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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="impala_kudu" rev="kudu">

  <title id="kudu">Using Impala to Query Kudu Tables</title>

  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Kudu"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Data Analysts"/>
      <data name="Category" value="Developers"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">Kudu</indexterm>
      You can use Impala to query tables stored by Apache Kudu. This capability
      allows convenient access to a storage system that is tuned for different kinds of
      workloads than the default with Impala.
    </p>

    <p>
      By default, Impala tables are stored on HDFS using data files with various file formats.
      HDFS files are ideal for bulk loads (append operations) and queries using full-table scans,
      but do not support in-place updates or deletes. Kudu is an alternative storage engine used
      by Impala which can do both in-place updates (for mixed read/write workloads) and fast scans
      (for data-warehouse/analytic operations). Using Kudu tables with Impala can simplify the
      ETL pipeline by avoiding extra steps to segregate and reorganize newly arrived data.
    </p>

    <p>
      Certain Impala SQL statements and clauses, such as <codeph>DELETE</codeph>,
      <codeph>UPDATE</codeph>, <codeph>UPSERT</codeph>, and <codeph>PRIMARY KEY</codeph> work
      only with Kudu tables. Other statements and clauses, such as <codeph>LOAD DATA</codeph>,
      <codeph>TRUNCATE TABLE</codeph>, and <codeph>INSERT OVERWRITE</codeph>, are not applicable
      to Kudu tables.
    </p>

    <p outputclass="toc inpage"/>

  </conbody>

  <concept id="kudu_benefits">

    <title>Benefits of Using Kudu Tables with Impala</title>

    <conbody>

      <p>
        The combination of Kudu and Impala works best for tables where scan performance is
        important, but data arrives continuously, in small batches, or needs to be updated
        without being completely replaced. HDFS-backed tables can require substantial overhead
        to replace or reorganize data files as new data arrives. Impala can perform efficient
        lookups and scans within Kudu tables, and Impala can also perform update or
        delete operations efficiently. You can also use the Kudu Java, C++, and Python APIs to
        do ingestion or transformation operations outside of Impala, and Impala can query the
        current data at any time.
      </p>

    </conbody>

  </concept>

  <concept id="kudu_config">

    <title>Configuring Impala for Use with Kudu</title>

    <conbody>

      <p>
        The <codeph>-kudu_master_hosts</codeph> configuration property must be set correctly
        for the <cmdname>impalad</cmdname> daemon, for <codeph>CREATE TABLE ... STORED AS
        KUDU</codeph> statements to connect to the appropriate Kudu server. Typically, the
        required value for this setting is <codeph><varname>kudu_host</varname>:7051</codeph>.
        In a high-availability Kudu deployment, specify the names of multiple Kudu hosts separated by commas.
      </p>

      <p>
        If the <codeph>-kudu_master_hosts</codeph> configuration property is not set, you can
        still associate the appropriate value for each table by specifying a
        <codeph>TBLPROPERTIES('kudu.master_addresses')</codeph> clause in the <codeph>CREATE TABLE</codeph> statement or
        changing the <codeph>TBLPROPERTIES('kudu.master_addresses')</codeph> value with an <codeph>ALTER TABLE</codeph>
        statement.
      </p>

    </conbody>

    <concept id="kudu_topology">

      <title>Cluster Topology for Kudu Tables</title>

      <conbody>

        <p>
          With HDFS-backed tables, you are typically concerned with the number of DataNodes in
          the cluster, how many and how large HDFS data files are read during a query, and
          therefore the amount of work performed by each DataNode and the network communication
          to combine intermediate results and produce the final result set.
        </p>

        <p>
          With Kudu tables, the topology considerations are different, because:
        </p>

        <ul>
          <li>
            <p>
              The underlying storage is managed and organized by Kudu, not represented as HDFS
              data files.
            </p>
          </li>

          <li>
            <p>
              Kudu handles some of the underlying mechanics of partitioning the data. You can specify
              the partitioning scheme with combinations of hash and range partitioning, so that you can
              decide how much effort to expend to manage the partitions as new data arrives. For example,
              you can construct partitions that apply to date ranges rather than a separate partition for each
              day or each hour.
            </p>
          </li>

          <li>
            <p> Data is physically divided based on units of storage called
                <term>tablets</term>. Tablets are stored by <term>tablet
                servers</term>. Each tablet server can store multiple tablets,
              and each tablet is replicated across multiple tablet servers,
              managed automatically by Kudu. Where practical, co-locate the
              tablet servers on the same hosts as the Impala daemons, although
              that is not required. </p>
          </li>
        </ul>

      </conbody>

    </concept>

  </concept>

  <concept id="kudu_replication_factor">
    <title>Kudu Replication Factor</title>
    <conbody>
      <p>
        By default, Kudu tables created through Impala use a tablet
        replication factor of 3. To change the replication factor for a Kudu
        table, specify the replication factor using <codeph>TBLPROPERTIES
          ('kudu.num_tablet_replicas' = '<i>n</i>')</codeph> in the <keyword
          keyref="create_table"/> statement.
      </p>

      <p>
        The number of replicas for a Kudu table must be odd.
      </p>

      <p> Altering the <codeph>kudu.num_tablet_replicas</codeph> property after
        table creation currently has no effect. </p>
    </conbody>
  </concept>

  <concept id="kudu_ddl">

    <title>Impala DDL Enhancements for Kudu Tables (CREATE TABLE and ALTER TABLE)</title>

    <prolog>
      <metadata>
        <data name="Category" value="DDL"/>
      </metadata>
    </prolog>

    <conbody>

      <p>
        You can use the Impala <codeph>CREATE TABLE</codeph> and <codeph>ALTER TABLE</codeph>
        statements to create and fine-tune the characteristics of Kudu tables. Because Kudu
        tables have features and properties that do not apply to other kinds of Impala tables,
        familiarize yourself with Kudu-related concepts and syntax first.
        For the general syntax of the <codeph>CREATE TABLE</codeph>
        statement for Kudu tables, see <xref keyref="create_table"/>.
      </p>

      <p outputclass="toc inpage"/>

    </conbody>

    <concept id="kudu_primary_key">

      <title>Primary Key Columns for Kudu Tables</title>

      <conbody>

        <p>
          Kudu tables introduce the notion of primary keys to Impala for the first time. The
          primary key is made up of one or more columns, whose values are combined and used as a
          lookup key during queries. The tuple represented by these columns must be unique and cannot contain any
          <codeph>NULL</codeph> values, and can never be updated once inserted. For a
          Kudu table, all the partition key columns must come from the set of
          primary key columns.
        </p>

        <p>
          The primary key has both physical and logical aspects:
        </p>

        <ul>
          <li>
            <p>
              On the physical side, it is used to map the data values to particular tablets for fast retrieval.
              Because the tuples formed by the primary key values are unique, the primary key columns are typically
              highly selective.
            </p>
          </li>
          <li>
            <p>
              On the logical side, the uniqueness constraint allows you to avoid duplicate data in a table.
              For example, if an <codeph>INSERT</codeph> operation fails partway through, only some of the
              new rows might be present in the table. You can re-run the same <codeph>INSERT</codeph>, and
              only the missing rows will be added. Or if data in the table is stale, you can run an
              <codeph>UPSERT</codeph> statement that brings the data up to date, without the possibility
              of creating duplicate copies of existing rows.
            </p>
          </li>
        </ul>

        <note>
          <p>
            Impala only allows <codeph>PRIMARY KEY</codeph> clauses and <codeph>NOT NULL</codeph>
            constraints on columns for Kudu tables. These constraints are enforced on the Kudu side.
          </p>
        </note>

      </conbody>

    </concept>

    <concept id="kudu_column_attributes" rev="IMPALA-3726">

      <title>Kudu-Specific Column Attributes for CREATE TABLE</title>

      <conbody>

        <p>
          For the general syntax of the <codeph>CREATE TABLE</codeph>
          statement for Kudu tables, see <xref keyref="create_table"/>.
          The following sections provide more detail for some of the
          Kudu-specific keywords you can use in column definitions.
        </p>

        <p>
          The column list in a <codeph>CREATE TABLE</codeph> statement can include the following
          attributes, which only apply to Kudu tables:
        </p>

<codeblock>
  PRIMARY KEY
| [NOT] NULL
| ENCODING <varname>codec</varname>
| COMPRESSION <varname>algorithm</varname>
| DEFAULT <varname>constant_expression</varname>
| BLOCK_SIZE <varname>number</varname>
</codeblock>

        <p outputclass="toc inpage">
          See the following sections for details about each column attribute.
        </p>

      </conbody>

      <concept id="kudu_primary_key_attribute">

        <title>PRIMARY KEY Attribute</title>

        <conbody>

          <p>
            The primary key for a Kudu table is a column, or set of columns, that uniquely
            identifies every row. The primary key value also is used as the natural sort order
            for the values from the table. The primary key value for each row is based on the
            combination of values for the columns.
          </p>

          <p conref="../shared/impala_common.xml#common/pk_implies_not_null"/>

          <p>
            The primary key columns must be the first ones specified in the <codeph>CREATE
            TABLE</codeph> statement. For a single-column primary key, you can include a
            <codeph>PRIMARY KEY</codeph> attribute inline with the column definition. For a
            multi-column primary key, you include a <codeph>PRIMARY KEY (<varname>c1</varname>,
            <varname>c2</varname>, ...)</codeph> clause as a separate entry at the end of the
            column list.
          </p>

          <p>
            You can specify the <codeph>PRIMARY KEY</codeph> attribute either inline in a single
            column definition, or as a separate clause at the end of the column list:
          </p>

<codeblock>
CREATE TABLE pk_inline
(
  col1 BIGINT PRIMARY KEY,
  col2 STRING,
  col3 BOOLEAN
) PARTITION BY HASH(col1) PARTITIONS 2 STORED AS KUDU;

CREATE TABLE pk_at_end
(
  col1 BIGINT,
  col2 STRING,
  col3 BOOLEAN,
  PRIMARY KEY (col1)
) PARTITION BY HASH(col1) PARTITIONS 2 STORED AS KUDU;
</codeblock>

          <p>
            When the primary key is a single column, these two forms are equivalent. If the
            primary key consists of more than one column, you must specify the primary key using
            a separate entry in the column list:
          </p>

<codeblock>
CREATE TABLE pk_multiple_columns
(
  col1 BIGINT,
  col2 STRING,
  col3 BOOLEAN,
  <b>PRIMARY KEY (col1, col2)</b>
) PARTITION BY HASH(col2) PARTITIONS 2 STORED AS KUDU;
</codeblock>

          <p>
            The <codeph>SHOW CREATE TABLE</codeph> statement always represents the
            <codeph>PRIMARY KEY</codeph> specification as a separate item in the column list:
          </p>

<codeblock>
CREATE TABLE inline_pk_rewritten (id BIGINT <b>PRIMARY KEY</b>, s STRING)
  PARTITION BY HASH(id) PARTITIONS 2 STORED AS KUDU;

SHOW CREATE TABLE inline_pk_rewritten;
+------------------------------------------------------------------------------+
| result                                                                       |
+------------------------------------------------------------------------------+
| CREATE TABLE user.inline_pk_rewritten (                                      |
|   id BIGINT NOT NULL ENCODING AUTO_ENCODING COMPRESSION DEFAULT_COMPRESSION, |
|   s STRING NULL ENCODING AUTO_ENCODING COMPRESSION DEFAULT_COMPRESSION,      |
|   <b>PRIMARY KEY (id)</b>                                                           |
| )                                                                            |
| PARTITION BY HASH (id) PARTITIONS 2                                          |
| STORED AS KUDU                                                               |
| TBLPROPERTIES ('kudu.master_addresses'='host.example.com')                   |
+------------------------------------------------------------------------------+
</codeblock>

          <p>
            The notion of primary key only applies to Kudu tables. Every Kudu table requires a
            primary key. The primary key consists of one or more columns. You must specify any
            primary key columns first in the column list.
          </p>

          <p>
            The contents of the primary key columns cannot be changed by an
            <codeph>UPDATE</codeph> or <codeph>UPSERT</codeph> statement. Including too many
            columns in the primary key (more than 5 or 6) can also reduce the performance of
            write operations. Therefore, pick the most selective and most frequently
            tested non-null columns for the primary key specification.
            If a column must always have a value, but that value
            might change later, leave it out of the primary key and use a <codeph>NOT
            NULL</codeph> clause for that column instead. If an existing row has an
            incorrect or outdated key column value, delete the old row and insert an entirely
            new row with the correct primary key.
          </p>

        </conbody>

      </concept>

      <concept id="kudu_not_null_attribute">

        <title>NULL | NOT NULL Attribute</title>

        <conbody>

          <p>
            For Kudu tables, you can specify which columns can contain nulls or not. This
            constraint offers an extra level of consistency enforcement for Kudu tables. If an
            application requires a field to always be specified, include a <codeph>NOT
            NULL</codeph> clause in the corresponding column definition, and Kudu prevents rows
            from being inserted with a <codeph>NULL</codeph> in that column.
          </p>

          <p>
            For example, a table containing geographic information might require the latitude
            and longitude coordinates to always be specified. Other attributes might be allowed
            to be <codeph>NULL</codeph>. For example, a location might not have a designated
            place name, its altitude might be unimportant, and its population might be initially
            unknown, to be filled in later.
          </p>

          <p conref="../shared/impala_common.xml#common/pk_implies_not_null"/>

          <p>
            For non-Kudu tables, Impala allows any column to contain <codeph>NULL</codeph>
            values, because it is not practical to enforce a <q>not null</q> constraint on HDFS
            data files that could be prepared using external tools and ETL processes.
          </p>

<codeblock>
CREATE TABLE required_columns
(
  id BIGINT PRIMARY KEY,
  latitude DOUBLE NOT NULL,
  longitude DOUBLE NOT NULL,
  place_name STRING,
  altitude DOUBLE,
  population BIGINT
) PARTITION BY HASH(id) PARTITIONS 2 STORED AS KUDU;
</codeblock>

          <p>
            During performance optimization, Kudu can use the knowledge that nulls are not
            allowed to skip certain checks on each input row, speeding up queries and join
            operations. Therefore, specify <codeph>NOT NULL</codeph> constraints when
            appropriate.
          </p>

          <p>
            The <codeph>NULL</codeph> clause is the default condition for all columns that are not
            part of the primary key. You can omit it, or specify it to clarify that you have made a
            conscious design decision to allow nulls in a column.
          </p>

          <p>
            Because primary key columns cannot contain any <codeph>NULL</codeph> values, the
            <codeph>NOT NULL</codeph> clause is not required for the primary key columns,
            but you might still specify it to make your code self-describing.
          </p>

        </conbody>

      </concept>

      <concept id="kudu_default_attribute">

        <title>DEFAULT Attribute</title>

        <conbody>

          <p>
            You can specify a default value for columns in Kudu tables. The default value can be
            any constant expression, for example, a combination of literal values, arithmetic
            and string operations. It cannot contain references to columns or non-deterministic
            function calls.
          </p>

          <p>
            The following example shows different kinds of expressions for the
            <codeph>DEFAULT</codeph> clause. The requirement to use a constant value means that
            you can fill in a placeholder value such as <codeph>NULL</codeph>, empty string,
            0, -1, <codeph>'N/A'</codeph> and so on, but you cannot reference functions or
            column names. Therefore, you cannot use <codeph>DEFAULT</codeph> to do things such as
            automatically making an uppercase copy of a string value, storing Boolean values based
            on tests of other columns, or add or subtract one from another column representing a sequence number.
          </p>

<codeblock>
CREATE TABLE default_vals
(
  id BIGINT PRIMARY KEY,
  name STRING NOT NULL DEFAULT 'unknown',
  address STRING DEFAULT upper('no fixed address'),
  age INT DEFAULT -1,
  earthling BOOLEAN DEFAULT TRUE,
  planet_of_origin STRING DEFAULT 'Earth',
  optional_col STRING DEFAULT NULL
) PARTITION BY HASH(id) PARTITIONS 2 STORED AS KUDU;
</codeblock>

          <note>
            <p>
              When designing an entirely new schema, prefer to use <codeph>NULL</codeph> as the
              placeholder for any unknown or missing values, because that is the universal convention
              among database systems. Null values can be stored efficiently, and easily checked with the
              <codeph>IS NULL</codeph> or <codeph>IS NOT NULL</codeph> operators. The <codeph>DEFAULT</codeph>
              attribute is appropriate when ingesting data that already has an established convention for
              representing unknown or missing values, or where the vast majority of rows have some common
              non-null value.
            </p>
          </note>

        </conbody>

      </concept>

      <concept id="kudu_encoding_attribute">

        <title>ENCODING Attribute</title>

        <conbody>

          <p>
            Each column in a Kudu table can optionally use an encoding, a low-overhead form of
            compression that reduces the size on disk, then requires additional CPU cycles to
            reconstruct the original values during queries. Typically, highly compressible data
            benefits from the reduced I/O to read the data back from disk.
          </p>

          <p>
            The encoding keywords that Impala recognizes are:

            <ul>
              <li>
                <p>
                  <codeph>AUTO_ENCODING</codeph>: use the default encoding based
                  on the column type, which are bitshuffle for the numeric type
                  columns and dictionary for the string type columns.
                </p>
              </li>
              <li>
                <p>
                  <codeph>PLAIN_ENCODING</codeph>: leave the value in its original binary format.
                </p>
              </li>
              <!-- GROUP_VARINT is internal use only, not documenting that although it shows up
                   in parser error messages. -->
              <li>
                <p>
                  <codeph>RLE</codeph>: compress repeated values (when sorted in primary key
                  order) by including a count.
                </p>
              </li>
              <li>
                <p>
                  <codeph>DICT_ENCODING</codeph>: when the number of different string values is
                  low, replace the original string with a numeric ID.
                </p>
              </li>
              <li>
                <p>
                  <codeph>BIT_SHUFFLE</codeph>: rearrange the bits of the values to efficiently
                  compress sequences of values that are identical or vary only slightly based
                  on primary key order. The resulting encoded data is also compressed with LZ4.
                </p>
              </li>
              <li>
                <p>
                  <codeph>PREFIX_ENCODING</codeph>: compress common prefixes in string values; mainly for use internally within Kudu.
                </p>
              </li>
            </ul>
          </p>

<!--
UNKNOWN, AUTO_ENCODING, PLAIN_ENCODING, PREFIX_ENCODING, GROUP_VARINT, RLE, DICT_ENCODING, BIT_SHUFFLE

No joy trying keywords UNKNOWN, or GROUP_VARINT with TINYINT and BIGINT.
-->

          <p>
            The following example shows the Impala keywords representing the encoding types.
            (The Impala keywords match the symbolic names used within Kudu.)
            For usage guidelines on the different kinds of encoding, see
            <xref href="https://kudu.apache.org/docs/schema_design.html" scope="external" format="html">the Kudu documentation</xref>.
            The <codeph>DESCRIBE</codeph> output shows how the encoding is reported after
            the table is created, and that omitting the encoding (in this case, for the
            <codeph>ID</codeph> column) is the same as specifying <codeph>DEFAULT_ENCODING</codeph>.
          </p>

<codeblock>
CREATE TABLE various_encodings
(
  id BIGINT PRIMARY KEY,
  c1 BIGINT ENCODING PLAIN_ENCODING,
  c2 BIGINT ENCODING AUTO_ENCODING,
  c3 TINYINT ENCODING BIT_SHUFFLE,
  c4 DOUBLE ENCODING BIT_SHUFFLE,
  c5 BOOLEAN ENCODING RLE,
  c6 STRING ENCODING DICT_ENCODING,
  c7 STRING ENCODING PREFIX_ENCODING
) PARTITION BY HASH(id) PARTITIONS 2 STORED AS KUDU;

-- Some columns are omitted from the output for readability.
describe various_encodings;
+------+---------+-------------+----------+-----------------+
| name | type    | primary_key | nullable | encoding        |
+------+---------+-------------+----------+-----------------+
| id   | bigint  | true        | false    | AUTO_ENCODING   |
| c1   | bigint  | false       | true     | PLAIN_ENCODING  |
| c2   | bigint  | false       | true     | AUTO_ENCODING   |
| c3   | tinyint | false       | true     | BIT_SHUFFLE     |
| c4   | double  | false       | true     | BIT_SHUFFLE     |
| c5   | boolean | false       | true     | RLE             |
| c6   | string  | false       | true     | DICT_ENCODING   |
| c7   | string  | false       | true     | PREFIX_ENCODING |
+------+---------+-------------+----------+-----------------+
</codeblock>

        </conbody>

      </concept>

      <concept id="kudu_compression_attribute">

        <title>COMPRESSION Attribute</title>

        <conbody>

          <p>
            You can specify a compression algorithm to use for each column in a Kudu table. This
            attribute imposes more CPU overhead when retrieving the values than the
            <codeph>ENCODING</codeph> attribute does. Therefore, use it primarily for columns with
            long strings that do not benefit much from the less-expensive <codeph>ENCODING</codeph>
            attribute.
          </p>

          <p>
            The choices for <codeph>COMPRESSION</codeph> are <codeph>LZ4</codeph>,
            <codeph>SNAPPY</codeph>, and <codeph>ZLIB</codeph>.
          </p>

          <note>
            <p>
              Columns that use the <codeph>BITSHUFFLE</codeph> encoding are already compressed
              using <codeph>LZ4</codeph>, and so typically do not need any additional
              <codeph>COMPRESSION</codeph> attribute.
            </p>
          </note>

          <p>
            The following example shows design considerations for several
            <codeph>STRING</codeph> columns with different distribution characteristics, leading
            to choices for both the <codeph>ENCODING</codeph> and <codeph>COMPRESSION</codeph>
            attributes. The <codeph>country</codeph> values come from a specific set of strings,
            therefore this column is a good candidate for dictionary encoding. The
            <codeph>post_id</codeph> column contains an ascending sequence of integers, where
            several leading bits are likely to be all zeroes, therefore this column is a good
            candidate for bitshuffle encoding. The <codeph>body</codeph>
            column and the corresponding columns for translated versions tend to be long unique
            strings that are not practical to use with any of the encoding schemes, therefore
            they employ the <codeph>COMPRESSION</codeph> attribute instead. The ideal compression
            codec in each case would require some experimentation to determine how much space
            savings it provided and how much CPU overhead it added, based on real-world data.
          </p>

<codeblock>
CREATE TABLE blog_posts
(
  user_id STRING ENCODING DICT_ENCODING,
  post_id BIGINT ENCODING BIT_SHUFFLE,
  subject STRING ENCODING PLAIN_ENCODING,
  body STRING COMPRESSION LZ4,
  spanish_translation STRING COMPRESSION SNAPPY,
  esperanto_translation STRING COMPRESSION ZLIB,
  PRIMARY KEY (user_id, post_id)
) PARTITION BY HASH(user_id, post_id) PARTITIONS 2 STORED AS KUDU;
</codeblock>

        </conbody>

      </concept>

      <concept id="kudu_block_size_attribute">

        <title>BLOCK_SIZE Attribute</title>

        <conbody>

          <p>
            Although Kudu does not use HDFS files internally, and thus is not affected by
            the HDFS block size, it does have an underlying unit of I/O called the
            <term>block size</term>. The <codeph>BLOCK_SIZE</codeph> attribute lets you set the
            block size for any column.
          </p>

          <p>
            The block size attribute is a relatively advanced feature. Refer to
            <xref href="https://kudu.apache.org/docs/index.html" scope="external" format="html">the Kudu documentation</xref>
            for usage details.
          </p>

<!-- Commenting out this example for the time being.
<codeblock>
CREATE TABLE performance_for_benchmark_xyz
(
  id BIGINT PRIMARY KEY,
  col1 BIGINT BLOCK_SIZE 4096,
  col2 STRING BLOCK_SIZE 16384,
  col3 SMALLINT BLOCK_SIZE 2048
) PARTITION BY HASH(id) PARTITIONS 2 STORED AS KUDU;
</codeblock>
-->

        </conbody>

      </concept>

    </concept>

    <concept id="kudu_partitioning">

      <title>Partitioning for Kudu Tables</title>

      <conbody>

        <p>
          Kudu tables use special mechanisms to distribute data among the underlying
          tablet servers. Although we refer to such tables as partitioned tables, they are
          distinguished from traditional Impala partitioned tables by use of different clauses
          on the <codeph>CREATE TABLE</codeph> statement. Kudu tables use
          <codeph>PARTITION BY</codeph>, <codeph>HASH</codeph>, <codeph>RANGE</codeph>, and
          range specification clauses rather than the <codeph>PARTITIONED BY</codeph> clause
          for HDFS-backed tables, which specifies only a column name and creates a new partition for each
          different value.
        </p>

        <p>
          For background information and architectural details about the Kudu partitioning
          mechanism, see
          <xref href="https://kudu.apache.org/kudu.pdf" scope="external" format="html">the Kudu white paper, section 3.2</xref>.
        </p>

<!-- Hiding but leaving in place for the moment, in case the white paper discussion isn't enough.
        <p>
          With Kudu tables, all of the columns involved in these clauses must be primary key
          columns. These clauses let you specify different ways to divide the data for each
          column, or even for different value ranges within a column. This flexibility lets you
          avoid problems with uneven distribution of data, where the partitioning scheme for
          HDFS tables might result in some partitions being much larger than others. By setting
          up an effective partitioning scheme for a Kudu table, you can ensure that the work for
          a query can be parallelized evenly across the hosts in a cluster.
        </p>
-->

        <note>
          <p>
            The Impala DDL syntax for Kudu tables is different than in early Kudu versions,
            which used an experimental fork of the Impala code. For example, the
            <codeph>DISTRIBUTE BY</codeph> clause is now <codeph>PARTITION BY</codeph>, the
            <codeph>INTO <varname>n</varname> BUCKETS</codeph> clause is now
            <codeph>PARTITIONS <varname>n</varname></codeph> and the range partitioning syntax
            is reworked to replace the <codeph>SPLIT ROWS</codeph> clause with more expressive
            syntax involving comparison operators.
          </p>
        </note>

        <p outputclass="toc inpage"/>

      </conbody>

      <concept id="kudu_hash_partitioning">
        <title>Hash Partitioning</title>
        <conbody>

          <p>
            Hash partitioning is the simplest type of partitioning for Kudu tables.
            For hash-partitioned Kudu tables, inserted rows are divided up between a fixed number
            of <q>buckets</q> by applying a hash function to the values of the columns specified
            in the <codeph>HASH</codeph> clause.
            Hashing ensures that rows with similar values are evenly distributed, instead of
            clumping together all in the same bucket. Spreading new rows across the buckets this
            way lets insertion operations work in parallel across multiple tablet servers.
            Separating the hashed values can impose additional overhead on queries, where
            queries with range-based predicates might have to read multiple tablets to retrieve
            all the relevant values.
          </p>

<codeblock>
-- 1M rows with 50 hash partitions = approximately 20,000 rows per partition.
-- The values in each partition are not sequential, but rather based on a hash function.
-- Rows 1, 99999, and 123456 might be in the same partition.
CREATE TABLE million_rows (id string primary key, s string)
  PARTITION BY HASH(id) PARTITIONS 50
  STORED AS KUDU;

-- Because the ID values are unique, we expect the rows to be roughly
-- evenly distributed between the buckets in the destination table.
INSERT INTO million_rows SELECT * FROM billion_rows ORDER BY id LIMIT 1e6;
</codeblock>

          <note>
            <p>
              The largest number of buckets that you can create with a <codeph>PARTITIONS</codeph>
              clause varies depending on the number of tablet servers in the cluster, while the smallest is 2.
              For simplicity, some of the simple <codeph>CREATE TABLE</codeph> statements throughout this section
              use <codeph>PARTITIONS 2</codeph> to illustrate the minimum requirements for a Kudu table.
              For large tables, prefer to use roughly 10 partitions per server in the cluster.
            </p>
          </note>

        </conbody>
      </concept>

      <concept id="kudu_range_partitioning">
        <title>Range Partitioning</title>
        <conbody>

          <p>
            Range partitioning lets you specify partitioning precisely, based on single values or ranges
            of values within one or more columns. You add one or more <codeph>RANGE</codeph> clauses to the
            <codeph>CREATE TABLE</codeph> statement, following the <codeph>PARTITION BY</codeph>
            clause.
          </p>

          <p>
            Range-partitioned Kudu tables use one or more range clauses, which include a
            combination of constant expressions, <codeph>VALUE</codeph> or <codeph>VALUES</codeph>
            keywords, and comparison operators. (This syntax replaces the <codeph>SPLIT
            ROWS</codeph> clause used with early Kudu versions.)
            For the full syntax, see <xref keyref="create_table"/>.
          </p>

<codeblock><![CDATA[
-- 50 buckets, all for IDs beginning with a lowercase letter.
-- Having only a single range enforces the allowed range of values
-- but does not add any extra parallelism.
create table million_rows_one_range (id string primary key, s string)
  partition by hash(id) partitions 50,
  range (partition 'a' <= values < '{')
  stored as kudu;

-- 50 buckets for IDs beginning with a lowercase letter
-- plus 50 buckets for IDs beginning with an uppercase letter.
-- Total number of buckets = number in the PARTITIONS clause x number of ranges.
-- We are still enforcing constraints on the primary key values
-- allowed in the table, and the 2 ranges provide better parallelism
-- as rows are inserted or the table is scanned.
create table million_rows_two_ranges (id string primary key, s string)
  partition by hash(id) partitions 50,
  range (partition 'a' <= values < '{', partition 'A' <= values < '[')
  stored as kudu;

-- Same as previous table, with an extra range covering the single key value '00000'.
create table million_rows_three_ranges (id string primary key, s string)
  partition by hash(id) partitions 50,
  range (partition 'a' <= values < '{', partition 'A' <= values < '[', partition value = '00000')
  stored as kudu;

-- The range partitioning can be displayed with a SHOW command in impala-shell.
show range partitions million_rows_three_ranges;
+---------------------+
| RANGE (id)          |
+---------------------+
| VALUE = "00000"     |
| "A" <= VALUES < "[" |
| "a" <= VALUES < "{" |
+---------------------+
]]>
</codeblock>

          <note>
            <p>
              When defining ranges, be careful to avoid <q>fencepost errors</q> where values at the
              extreme ends might be included or omitted by accident. For example, in the tables defined
              in the preceding code listings, the range <codeph><![CDATA["a" <= VALUES < "{"]]></codeph> ensures that
              any values starting with <codeph>z</codeph>, such as <codeph>za</codeph> or <codeph>zzz</codeph>
              or <codeph>zzz-ZZZ</codeph>, are all included, by using a less-than operator for the smallest
              value after all the values starting with <codeph>z</codeph>.
            </p>
          </note>

          <p>
            For range-partitioned Kudu tables, an appropriate range must exist before a data value can be created in the table.
            Any <codeph>INSERT</codeph>, <codeph>UPDATE</codeph>, or <codeph>UPSERT</codeph> statements fail if they try to
            create column values that fall outside the specified ranges. The error checking for ranges is performed on the
            Kudu side; Impala passes the specified range information to Kudu, and passes back any error or warning if the
            ranges are not valid. (A nonsensical range specification causes an error for a DDL statement, but only a warning
            for a DML statement.)
          </p>

          <p>
            Ranges can be non-contiguous:
          </p>

<codeblock><![CDATA[
partition by range (year) (partition 1885 <= values <= 1889, partition 1893 <= values <= 1897)

partition by range (letter_grade) (partition value = 'A', partition value = 'B',
  partition value = 'C', partition value = 'D', partition value = 'F')
]]>
</codeblock>

          <p>
            The <codeph>ALTER TABLE</codeph> statement with the <codeph>ADD PARTITION</codeph> or
            <codeph>DROP PARTITION</codeph> clauses can be used to add or remove ranges from an
            existing Kudu table.
          </p>

<codeblock><![CDATA[
ALTER TABLE foo ADD PARTITION 30 <= VALUES < 50;
ALTER TABLE foo DROP PARTITION 1 <= VALUES < 5;
]]>
</codeblock>

          <p>
            When a range is added, the new range must not overlap with any of the previous ranges;
            that is, it can only fill in gaps within the previous ranges.
          </p>

<codeblock><![CDATA[
alter table test_scores add range partition value = 'E';

alter table year_ranges add range partition 1890 <= values < 1893;
]]>
</codeblock>

          <p>
            When a range is removed, all the associated rows in the table are deleted. (This
            is true whether the table is internal or external.)
          </p>

<codeblock><![CDATA[
alter table test_scores drop range partition value = 'E';

alter table year_ranges drop range partition 1890 <= values < 1893;
]]>
</codeblock>

        <p>
          Kudu tables can also use a combination of hash and range partitioning.
        </p>

<codeblock><![CDATA[
partition by hash (school) partitions 10,
  range (letter_grade) (partition value = 'A', partition value = 'B',
    partition value = 'C', partition value = 'D', partition value = 'F')
]]>
</codeblock>

        </conbody>
      </concept>

      <concept id="kudu_partitioning_misc">
        <title>Working with Partitioning in Kudu Tables</title>
        <conbody>

          <p>
            To see the current partitioning scheme for a Kudu table, you can use the <codeph>SHOW
            CREATE TABLE</codeph> statement or the <codeph>SHOW PARTITIONS</codeph> statement. The
            <codeph>CREATE TABLE</codeph> syntax displayed by this statement includes all the
            hash, range, or both clauses that reflect the original table structure plus any
            subsequent <codeph>ALTER TABLE</codeph> statements that changed the table structure.
          </p>

          <p>
            To see the underlying buckets and partitions for a Kudu table, use the
            <codeph>SHOW TABLE STATS</codeph> or <codeph>SHOW PARTITIONS</codeph> statement.
          </p>

        </conbody>
      </concept>

    </concept>

    <concept id="kudu_timestamps">

      <title>Handling Date, Time, or Timestamp Data with Kudu</title>

      <conbody>

        <p conref="../shared/impala_common.xml#common/kudu_timestamp_details"/>

<codeblock rev="2.9.0 IMPALA-5137"><![CDATA[--- Make a table representing a date/time value as TIMESTAMP.
-- The strings representing the partition bounds are automatically
-- cast to TIMESTAMP values.
create table native_timestamp(id bigint, when_exactly timestamp, event string, primary key (id, when_exactly))
  partition by hash (id) partitions 20,
  range (when_exactly)
  (
    partition '2015-01-01' <= values < '2016-01-01',
    partition '2016-01-01' <= values < '2017-01-01',
    partition '2017-01-01' <= values < '2018-01-01'
  )
  stored as kudu;

insert into native_timestamp values (12345, now(), 'Working on doc examples');

select * from native_timestamp;
+-------+-------------------------------+-------------------------+
| id    | when_exactly                  | event                   |
+-------+-------------------------------+-------------------------+
| 12345 | 2017-05-31 16:27:42.667542000 | Working on doc examples |
+-------+-------------------------------+-------------------------+
]]>
</codeblock>

        <p>
          Because Kudu tables have some performance overhead to convert <codeph>TIMESTAMP</codeph>
          columns to the Impala 96-bit internal representation, for performance-critical
          applications you might store date/time information as the number
          of seconds, milliseconds, or microseconds since the Unix epoch date of January 1,
          1970. Specify the column as <codeph>BIGINT</codeph> in the Impala <codeph>CREATE
          TABLE</codeph> statement, corresponding to an 8-byte integer (an
          <codeph>int64</codeph>) in the underlying Kudu table). Then use Impala date/time
          conversion functions as necessary to produce a numeric, <codeph>TIMESTAMP</codeph>,
          or <codeph>STRING</codeph> value depending on the context.
        </p>

        <p>
          For example, the <codeph>unix_timestamp()</codeph> function returns an integer result
          representing the number of seconds past the epoch. The <codeph>now()</codeph> function
          produces a <codeph>TIMESTAMP</codeph> representing the current date and time, which can
          be passed as an argument to <codeph>unix_timestamp()</codeph>. And string literals
          representing dates and date/times can be cast to <codeph>TIMESTAMP</codeph>, and from there
          converted to numeric values. The following examples show how you might store a date/time
          column as <codeph>BIGINT</codeph> in a Kudu table, but still use string literals and
          <codeph>TIMESTAMP</codeph> values for convenience.
        </p>

<codeblock><![CDATA[
-- now() returns a TIMESTAMP and shows the format for string literals you can cast to TIMESTAMP.
select now();
+-------------------------------+
| now()                         |
+-------------------------------+
| 2017-01-25 23:50:10.132385000 |
+-------------------------------+

-- unix_timestamp() accepts either a TIMESTAMP or an equivalent string literal.
select unix_timestamp(now());
+------------------+
| unix_timestamp() |
+------------------+
| 1485386670       |
+------------------+

select unix_timestamp('2017-01-01');
+------------------------------+
| unix_timestamp('2017-01-01') |
+------------------------------+
| 1483228800                   |
+------------------------------+

-- Make a table representing a date/time value as BIGINT.
-- Construct 1 range partition and 20 associated hash partitions for each year.
-- Use date/time conversion functions to express the ranges as human-readable dates.
create table time_series(id bigint, when_exactly bigint, event string, primary key (id, when_exactly))
  partition by hash (id) partitions 20,
  range (when_exactly)
  (
    partition unix_timestamp('2015-01-01') <= values < unix_timestamp('2016-01-01'),
    partition unix_timestamp('2016-01-01') <= values < unix_timestamp('2017-01-01'),
    partition unix_timestamp('2017-01-01') <= values < unix_timestamp('2018-01-01')
  )
  stored as kudu;

-- On insert, we can transform a human-readable date/time into a numeric value.
insert into time_series values (12345, unix_timestamp('2017-01-25 23:24:56'), 'Working on doc examples');

-- On retrieval, we can examine the numeric date/time value or turn it back into a string for readability.
select id, when_exactly, from_unixtime(when_exactly) as 'human-readable date/time', event
  from time_series order by when_exactly limit 100;
+-------+--------------+--------------------------+-------------------------+
| id    | when_exactly | human-readable date/time | event                   |
+-------+--------------+--------------------------+-------------------------+
| 12345 | 1485386696   | 2017-01-25 23:24:56      | Working on doc examples |
+-------+--------------+--------------------------+-------------------------+
]]>
</codeblock>

        <note>
          <p>
            If you do high-precision arithmetic involving numeric date/time values,
            when dividing millisecond values by 1000, or microsecond values by 1 million, always
            cast the integer numerator to a <codeph>DECIMAL</codeph> with sufficient precision
            and scale to avoid any rounding or loss of precision.
          </p>
        </note>

<codeblock><![CDATA[
-- 1 million and 1 microseconds = 1.000001 seconds.
select microseconds,
  cast (microseconds as decimal(20,7)) / 1e6 as fractional_seconds
  from table_with_microsecond_column;
+--------------+----------------------+
| microseconds | fractional_seconds   |
+--------------+----------------------+
| 1000001      | 1.000001000000000000 |
+--------------+----------------------+
]]>
</codeblock>

      </conbody>

    </concept>

    <concept id="kudu_metadata">

      <title>How Impala Handles Kudu Metadata</title>

      <conbody>
        <note>This section only applies the Kudu services that are not
          integrated with the Hive Metastore (HMS).</note>
        <p conref="../shared/impala_common.xml#common/kudu_metadata_intro"/>
        <p conref="../shared/impala_common.xml#common/kudu_metadata_details"/>
        <p> Because Kudu manages the metadata for its own tables separately from
          the metastore database, there is a table name stored in the metastore
          database for Impala to use, and a table name on the Kudu side, and
          these names can be modified independently through <codeph>ALTER
            TABLE</codeph> statements. </p>
        <p> To avoid potential name conflicts, the prefix
            <codeph>impala::</codeph> and the Impala database name are encoded
          into the underlying Kudu table name: </p>
        <codeblock><![CDATA[
create database some_database;
use some_database;

create table table_name_demo (x int primary key, y int)
  partition by hash (x) partitions 2 stored as kudu;

describe formatted table_name_demo;
...
kudu.table_name  | impala::some_database.table_name_demo
]]>
</codeblock>
        <p> See <xref keyref="kudu_tables"/> for examples of how to change the
          name of the Impala table in the metastore database, the name of the
          underlying Kudu table, or both. </p>
      </conbody>

    </concept>

  </concept>
  <concept id="kudu_hms">
    <title>Working with Kudu Integrated with Hive Metastore</title>
    <conbody>
      <p>Starting from Kudu 1.10 and Impala 3.3, Impala supports Kudu services
        integrated with the Hive Metastore (HMS). See <xref
          href="https://kudu.apache.org/docs/hive_metastore.html#hive_metastore"
          format="html" scope="external">the HMS integration
          documentation</xref> for more details on Kudu’s Hive Metastore
        integration.</p>
      <p>The following are some of the changes you need to consider when working
        with Kudu services integrated with the HMS.<ul>
          <li> When Kudu is integrated with the Hive Metastore, Impala must be
            configured to use the same HMS as Kudu.</li>
          <li> Since there may be no one-to-one mapping between Kudu tables and
            external tables, only internal tables are automatically
            synchronized. </li>
          <li>When you create a table in Kudu, Kudu will create an HMS entry for
            that table with the internal table type.</li>
          <li>When the Kudu service is integrated with the HMS, internal table
            entries will be created automatically in the HMS when tables are
            created in Kudu without Impala. To access these tables through
            Impala, run <codeph>INVALIDATE METADATA</codeph> statement so Impala
            picks up the latest metadata.</li>
        </ul></p>
    </conbody>
  </concept>

  <concept id="kudu_etl">

    <title>Loading Data into Kudu Tables</title>

    <conbody>

      <p>
        Kudu tables are well-suited to use cases where data arrives continuously, in small or
        moderate volumes. To bring data into Kudu tables, use the Impala <codeph>INSERT</codeph>
        and <codeph>UPSERT</codeph> statements. The <codeph>LOAD DATA</codeph> statement does
        not apply to Kudu tables.
      </p>

      <p>
        Because Kudu manages its own storage layer that is optimized for smaller block sizes than
        HDFS, and performs its own housekeeping to keep data evenly distributed, it is not
        subject to the <q>many small files</q> issue and does not need explicit reorganization
        and compaction as the data grows over time. The partitions within a Kudu table can be
        specified to cover a variety of possible data distributions, instead of hardcoding a new
        partition for each new day, hour, and so on, which can lead to inefficient,
        hard-to-scale, and hard-to-manage partition schemes with HDFS tables.
      </p>

      <p>
        Your strategy for performing ETL or bulk updates on Kudu tables should take into account
        the limitations on consistency for DML operations.
      </p>

      <p>
        Make <codeph>INSERT</codeph>, <codeph>UPDATE</codeph>, and <codeph>UPSERT</codeph>
        operations <term>idempotent</term>: that is, able to be applied multiple times and still
        produce an identical result.
      </p>

      <p>
        If a bulk operation is in danger of exceeding capacity limits due to timeouts or high
        memory usage, split it into a series of smaller operations.
      </p>

      <p>
        Avoid running concurrent ETL operations where the end results depend on precise
        ordering. In particular, do not rely on an <codeph>INSERT ... SELECT</codeph> statement
        that selects from the same table into which it is inserting, unless you include extra
        conditions in the <codeph>WHERE</codeph> clause to avoid reading the newly inserted rows
        within the same statement.
      </p>

      <p>
        Because relationships between tables cannot be enforced by Impala and Kudu, and cannot
        be committed or rolled back together, do not expect transactional semantics for
        multi-table operations.
      </p>

    </conbody>

  </concept>

  <concept id="kudu_dml">

    <title>Impala DML Support for Kudu Tables (INSERT, UPDATE, DELETE, UPSERT)</title>

    <prolog>
      <metadata>
        <data name="Category" value="DML"/>
      </metadata>
    </prolog>

    <conbody>

      <p>
        Impala supports certain DML statements for Kudu tables only. The <codeph>UPDATE</codeph>
        and <codeph>DELETE</codeph> statements let you modify data within Kudu tables without
        rewriting substantial amounts of table data. The <codeph>UPSERT</codeph> statement acts
        as a combination of <codeph>INSERT</codeph> and <codeph>UPDATE</codeph>, inserting rows
        where the primary key does not already exist, and updating the non-primary key columns
        where the primary key does already exist in the table.
      </p>

      <p>
        The <codeph>INSERT</codeph> statement for Kudu tables honors the unique and <codeph>NOT
        NULL</codeph> requirements for the primary key columns.
      </p>

      <p>
        Because Impala and Kudu do not support transactions, the effects of any
        <codeph>INSERT</codeph>, <codeph>UPDATE</codeph>, or <codeph>DELETE</codeph> statement
        are immediately visible. For example, you cannot do a sequence of
        <codeph>UPDATE</codeph> statements and only make the changes visible after all the
        statements are finished. Also, if a DML statement fails partway through, any rows that
        were already inserted, deleted, or changed remain in the table; there is no rollback
        mechanism to undo the changes.
      </p>

      <p>
        In particular, an <codeph>INSERT ... SELECT</codeph> statement that refers to the table
        being inserted into might insert more rows than expected, because the
        <codeph>SELECT</codeph> part of the statement sees some of the new rows being inserted
        and processes them again.
      </p>

      <note>
        <p>
          The <codeph>LOAD DATA</codeph> statement, which involves manipulation of HDFS data files,
          does not apply to Kudu tables.
        </p>
      </note>

      <p conref="../shared/impala_common.xml#common/kudu_hints"/>

    </conbody>

  </concept>

  <concept id="kudu_consistency">

    <title>Consistency Considerations for Kudu Tables</title>

    <conbody>

      <p>
        Kudu tables have consistency characteristics such as uniqueness, controlled by the
        primary key columns, and non-nullable columns. The emphasis for consistency is on
        preventing duplicate or incomplete data from being stored in a table.
      </p>

      <p>
        Currently, Kudu does not enforce strong consistency for order of operations, total
        success or total failure of a multi-row statement, or data that is read while a write
        operation is in progress. Changes are applied atomically to each row, but not applied
        as a single unit to all rows affected by a multi-row DML statement. That is, Kudu does
        not currently have atomic multi-row statements or isolation between statements.
      </p>

      <p>
        If some rows are rejected during a DML operation because of a mismatch with duplicate
        primary key values, <codeph>NOT NULL</codeph> constraints, and so on, the statement
        succeeds with a warning. Impala still inserts, deletes, or updates the other rows that
        are not affected by the constraint violation.
      </p>

      <p>
        Consequently, the number of rows affected by a DML operation on a Kudu table might be
        different than you expect.
      </p>

      <p>
        Because there is no strong consistency guarantee for information being inserted into,
        deleted from, or updated across multiple tables simultaneously, consider denormalizing
        the data where practical. That is, if you run separate <codeph>INSERT</codeph>
        statements to insert related rows into two different tables, one <codeph>INSERT</codeph>
        might fail while the other succeeds, leaving the data in an inconsistent state. Even if
        both inserts succeed, a join query might happen during the interval between the
        completion of the first and second statements, and the query would encounter incomplete
        inconsistent data. Denormalizing the data into a single wide table can reduce the
        possibility of inconsistency due to multi-table operations.
      </p>

      <p>
        Information about the number of rows affected by a DML operation is reported in
        <cmdname>impala-shell</cmdname> output, and in the <codeph>PROFILE</codeph> output, but
        is not currently reported to HiveServer2 clients such as JDBC or ODBC applications.
      </p>

    </conbody>

  </concept>

  <concept id="kudu_security">

    <title>Security Considerations for Kudu Tables</title>

    <conbody>

      <p>
        Security for Kudu tables involves:
      </p>

      <ul>
        <li>
          <p>
            Sentry authorization.
          </p>
          <p conref="../shared/impala_common.xml#common/kudu_sentry_limitations"/>
        </li>

        <li rev="2.9.0">
          <p>
            Kerberos authentication. See <xref keyref="kudu_security"/> for details.
          </p>
        </li>

        <li rev="2.9.0">
          <p>
            TLS encryption. See <xref keyref="kudu_security"/> for details.
          </p>
        </li>

        <li>
          <p>
            Lineage tracking.
          </p>
        </li>

        <li>
          <p>
            Auditing.
          </p>
        </li>

        <li>
          <p>
            Redaction of sensitive information from log files.
          </p>
        </li>
      </ul>

    </conbody>

  </concept>

  <concept id="kudu_performance">

    <title>Impala Query Performance for Kudu Tables</title>

    <conbody>

      <p>
        For queries involving Kudu tables, Impala can delegate much of the work of filtering the
        result set to Kudu, avoiding some of the I/O involved in full table scans of tables
        containing HDFS data files. This type of optimization is especially effective for
        partitioned Kudu tables, where the Impala query <codeph>WHERE</codeph> clause refers to
        one or more primary key columns that are also used as partition key columns. For
        example, if a partitioned Kudu table uses a <codeph>HASH</codeph> clause for
        <codeph>col1</codeph> and a <codeph>RANGE</codeph> clause for <codeph>col2</codeph>, a
        query using a clause such as <codeph>WHERE col1 IN (1,2,3) AND col2 &gt; 100</codeph>
        can determine exactly which tablet servers contain relevant data, and therefore
        parallelize the query very efficiently.
      </p>

      <p rev="2.11.0 IMPALA-4252">
        In <keyword keyref="impala211_full"/> and higher, Impala can push down additional
        information to optimize join queries involving Kudu tables. If the join clause
        contains predicates of the form
        <codeph><varname>column</varname> = <varname>expression</varname></codeph>,
        after Impala constructs a hash table of possible matching values for the
        join columns from the bigger table (either an HDFS table or a Kudu table), Impala
        can <q>push down</q> the minimum and maximum matching column values to Kudu,
        so that Kudu can more efficiently locate matching rows in the second (smaller) table.
        These min/max filters are affected by the <codeph>RUNTIME_FILTER_MODE</codeph>,
        <codeph>RUNTIME_FILTER_WAIT_TIME_MS</codeph>, and <codeph>DISABLE_ROW_RUNTIME_FILTERING</codeph>
        query options; the min/max filters are not affected by the
        <codeph>RUNTIME_BLOOM_FILTER_SIZE</codeph>, <codeph>RUNTIME_FILTER_MIN_SIZE</codeph>,
        <codeph>RUNTIME_FILTER_MAX_SIZE</codeph>, and <codeph>MAX_NUM_RUNTIME_FILTERS</codeph>
        query options.
      </p>

      <p>
        See <xref keyref="explain"/> for examples of evaluating the effectiveness of
        the predicate pushdown for a specific query against a Kudu table.
      </p>

      <p conref="../shared/impala_common.xml#common/tablesample_caveat"/>

      <!-- Hide until subtopics are ready to display. -->
      <p outputclass="toc inpage" audience="hidden"/>

    </conbody>

    <concept id="kudu_vs_parquet" audience="hidden">
    <!-- To do: if there is enough real-world experience in future to have a
         substantive discussion of this subject, revisit this topic and
         consider unhiding it. -->

      <title>How Kudu Works with Column-Oriented Operations</title>

      <conbody>

        <p>
          For immutable data, Impala is often used with Parquet tables due to the efficiency of
          the column-oriented Parquet layout. This section describes how Kudu stores and
          retrieves columnar data, to help you understand performance and storage considerations
          of Kudu tables as compared with Parquet tables.
        </p>

      </conbody>

    </concept>

    <concept id="kudu_memory" audience="hidden">
    <!-- To do: if there is enough real-world experience in future to have a
         substantive discussion of this subject, revisit this topic and
         consider unhiding it. -->

      <title>Memory Usage for Operations on Kudu Tables</title>

      <conbody>

        <p>
          The Apache Kudu architecture, topology, and data storage techniques result in
          different patterns of memory usage for Impala statements than with HDFS-backed tables.
        </p>

      </conbody>

    </concept>

  </concept>

</concept>