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

  <title>Optimizer Hints</title>

  <titlealts audience="PDF">

    <navtitle>Optimizer Hints</navtitle>

  </titlealts>

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

  <conbody>

    <p>
      <indexterm audience="hidden">hints</indexterm>
      The Impala SQL supports query hints, for fine-tuning the inner workings of queries.
      Specify hints as a temporary workaround for expensive queries, where missing statistics or
      other factors cause inefficient performance.
    </p>

    <p>
      Hints are most often used for the resource-intensive Impala queries, such as:
    </p>

    <ul>
      <li>
        Join queries involving large tables, where intermediate result sets are transmitted
        across the network to evaluate the join conditions.
      </li>

      <li>
        Inserting into partitioned Parquet tables, where many memory buffers could be allocated
        on each host to hold intermediate results for each partition.
      </li>
    </ul>

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

    <p rev="2.0.0">
      In <keyword keyref="impala20_full"/> and higher, you can specify the hints inside comments
      that use either the <codeph>/* */</codeph> or <codeph>--</codeph> notation. Specify a
      <codeph>+</codeph> symbol immediately before the hint name. Recently added hints are only
      available using the <codeph>/* */</codeph> and <codeph>--</codeph> notation. For clarity,
      the <codeph>/* */</codeph> and <codeph>--</codeph> styles are used in the syntax and
      examples throughout this section. With the <codeph>/* */</codeph> or <codeph>--</codeph>
      notation for hints, specify a <codeph>+</codeph> symbol immediately before the first hint
      name. Multiple hints can be specified separated by commas, for example <codeph>/*
      +clustered,shuffle */</codeph>
    </p>

<codeblock rev="2.0.0">SELECT STRAIGHT_JOIN <varname>select_list</varname> FROM
<varname>join_left_hand_table</varname>
  JOIN /* +BROADCAST|SHUFFLE */
<varname>join_right_hand_table</varname>
<varname>remainder_of_query</varname>;

SELECT <varname>select_list</varname> FROM
<varname>join_left_hand_table</varname>
  JOIN -- +BROADCAST|SHUFFLE
<varname>join_right_hand_table</varname>
<varname>remainder_of_query</varname>;

INSERT <varname>insert_clauses</varname>
  /* +SHUFFLE|NOSHUFFLE */
  SELECT <varname>remainder_of_query</varname>;

INSERT <varname>insert_clauses</varname>
  -- +SHUFFLE|NOSHUFFLE
  SELECT <varname>remainder_of_query</varname>;

<ph rev="2.12.0 IMPALA-4168">
INSERT /* +SHUFFLE|NOSHUFFLE */
  <varname>insert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;</ph>

<ph rev="2.12.0 IMPALA-4168">
INSERT -- +SHUFFLE|NOSHUFFLE
  <varname>insert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;</ph>

<ph rev="2.12.0 IMPALA-4168">
UPSERT /* +SHUFFLE|NOSHUFFLE */
  <varname>upsert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;</ph>

<ph rev="2.12.0 IMPALA-4168">
UPSERT -- +SHUFFLE|NOSHUFFLE
  <varname>upsert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;</ph>

<ph rev="IMPALA-2924">SELECT <varname>select_list</varname> FROM
<varname>table_ref</varname>
  /* +{SCHEDULE_CACHE_LOCAL | SCHEDULE_DISK_LOCAL | SCHEDULE_REMOTE}
    [,RANDOM_REPLICA] */
<varname>remainder_of_query</varname>;</ph>

<ph rev="IMPALA-2522 2.8.0">INSERT <varname>insert_clauses</varname>
  -- +CLUSTERED
  SELECT <varname>remainder_of_query</varname>;

INSERT <varname>insert_clauses</varname>
  /* +CLUSTERED */
  SELECT <varname>remainder_of_query</varname>;</ph>

<ph rev="2.12.0 IMPALA-4168">INSERT -- +CLUSTERED
  <varname>insert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;

INSERT /* +CLUSTERED */
  <varname>insert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;

UPSERT -- +CLUSTERED
  <varname>upsert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;

UPSERT /* +CLUSTERED */
  <varname>upsert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;</ph>

CREATE /* +SHUFFLE|NOSHUFFLE */
  <varname>table_clauses</varname>
  AS SELECT <varname>remainder_of_query</varname>;

CREATE -- +SHUFFLE|NOSHUFFLE
  <varname>table_clauses</varname>
  AS SELECT <varname>remainder_of_query</varname>;

CREATE /* +CLUSTER|NOCLUSTER */
  <varname>table_clauses</varname>
  AS SELECT <varname>remainder_of_query</varname>;

CREATE -- +CLUSTER|NOCLUSTER
  <varname>table_clauses</varname>
  AS SELECT <varname>remainder_of_query</varname>;
</codeblock>

    <p>
      The square bracket style hints are supported for backward compatibility, but the syntax is
      deprecated and will be removed in a future release. For that reason, any newly added hints
      are not available with the square bracket syntax.
    </p>

<codeblock>SELECT STRAIGHT_JOIN <varname>select_list</varname> FROM
<varname>join_left_hand_table</varname>
  JOIN [{ /* +BROADCAST */ | /* +SHUFFLE */ }]
<varname>join_right_hand_table</varname>
<varname>remainder_of_query</varname>;

INSERT <varname>insert_clauses</varname>
  [{ /* +SHUFFLE */ | /* +NOSHUFFLE */ }]
  [<ph rev="IMPALA-2522 2.8.0">/* +CLUSTERED */</ph>]
  SELECT <varname>remainder_of_query</varname>;

<ph rev="2.12.0 IMPALA-4168">
UPSERT [{ /* +SHUFFLE */ | /* +NOSHUFFLE */ }]
  [<ph rev="IMPALA-2522 2.8.0">/* +CLUSTERED */</ph>]
  <varname>upsert_clauses</varname>
  SELECT <varname>remainder_of_query</varname>;</ph>
</codeblock>

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

    <p>
      With both forms of hint syntax, include the <codeph>STRAIGHT_JOIN</codeph> keyword
      immediately after the <codeph>SELECT</codeph> and any <codeph>DISTINCT</codeph> or
      <codeph>ALL</codeph> keywords to prevent Impala from reordering the tables in a way that
      makes the join-related hints ineffective.
    </p>

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

    <p>
      To reduce the need to use hints, run the <codeph>COMPUTE STATS</codeph> statement against
      all tables involved in joins, or used as the source tables for <codeph>INSERT ...
      SELECT</codeph> operations where the destination is a partitioned Parquet table. Do this
      operation after loading data or making substantial changes to the data within each table.
      Having up-to-date statistics helps Impala choose more efficient query plans without the
      need for hinting. See <xref href="impala_perf_stats.xml#perf_stats"/> for details and
      examples.
    </p>

    <p>
      To see which join strategy is used for a particular query, examine the
      <codeph>EXPLAIN</codeph> output for that query. See
      <xref href="impala_explain_plan.xml#perf_explain"/> for details and examples.
    </p>

    <p>
      <b>Hints for join queries:</b>
    </p>

    <p>
      The <codeph>/* +BROADCAST */</codeph> and <codeph>/* +SHUFFLE */</codeph> hints control
      the execution strategy for join queries. Specify one of the following constructs
      immediately after the <codeph>JOIN</codeph> keyword in a query:
    </p>

    <ul>
      <li>
        <codeph>/* +SHUFFLE */</codeph> makes that join operation use the <q>partitioned</q>
        technique, which divides up corresponding rows from both tables using a hashing
        algorithm, sending subsets of the rows to other nodes for processing. (The keyword
        <codeph>SHUFFLE</codeph> is used to indicate a <q>partitioned join</q>, because that
        type of join is not related to <q>partitioned tables</q>.) Since the alternative
        <q>broadcast</q> join mechanism is the default when table and index statistics are
        unavailable, you might use this hint for queries where broadcast joins are unsuitable;
        typically, partitioned joins are more efficient for joins between large tables of
        similar size.
      </li>

      <li>
        <codeph>/* +BROADCAST */</codeph> makes that join operation use the <q>broadcast</q>
        technique that sends the entire contents of the right-hand table to all nodes involved
        in processing the join. This is the default mode of operation when table and index
        statistics are unavailable, so you would typically only need it if stale metadata caused
        Impala to mistakenly choose a partitioned join operation. Typically, broadcast joins are
        more efficient in cases where one table is much smaller than the other. (Put the smaller
        table on the right side of the <codeph>JOIN</codeph> operator.)
      </li>
    </ul>

    <p>
      <b>Hints for INSERT ... SELECT and CREATE TABLE AS SELECT (CTAS):</b>
    </p>

    <p id="insert_hints">
      When inserting into partitioned tables, such as using the Parquet file format, you can
      include a hint in the <codeph>INSERT</codeph> or <codeph>CREATE TABLE AS
      SELECT(CTAS)</codeph> statements to fine-tune the overall performance of the operation and
      its resource usage.
    </p>

    <p>
      You would only use hints if an <codeph>INSERT</codeph> or <codeph>CTAS</codeph> into a
      partitioned table was failing due to capacity limits, or if such an operation was
      succeeding but with less-than-optimal performance.
    </p>

    <ul>
      <li>
        <codeph>/* +SHUFFLE */</codeph> and <codeph>/* +NOSHUFFLE */</codeph> Hints
        <ul>
          <li>
            <codeph>/* +SHUFFLE */</codeph> adds an exchange node, before writing the data,
            which re-partitions the result of the <codeph>SELECT</codeph> based on the
            partitioning columns of the target table. With this hint, only one node writes to a
            partition at a time, minimizing the global number of simultaneous writes and the
            number of memory buffers holding data for individual partitions. This also reduces
            fragmentation, resulting in fewer files. Thus it reduces overall resource usage of
            the <codeph>INSERT</codeph> or <codeph>CTAS</codeph> operation and allows some
            operations to succeed that otherwise would fail. It does involve some data transfer
            between the nodes so that the data files for a particular partition are all written
            on the same node.
            <p>
              Use <codeph>/* +SHUFFLE */</codeph> in cases where an <codeph>INSERT</codeph> or
              <codeph>CTAS</codeph> statement fails or runs inefficiently due to all nodes
              attempting to write data for all partitions.
            </p>

            <p>
              If the table is unpartitioned or every partitioning expression is constant, then
              <codeph>/* +SHUFFLE */</codeph> will cause every write to happen on the
              coordinator node.
            </p>
          </li>

          <li>
            <codeph>/* +NOSHUFFLE */</codeph> does not add exchange node before inserting to
            partitioned tables and disables re-partitioning. So the selected execution plan
            might be faster overall, but might also produce a larger number of small data files
            or exceed capacity limits, causing the <codeph>INSERT</codeph> or
            <codeph>CTAS</codeph> operation to fail.
            <p>
              Impala automatically uses the <codeph>/* +SHUFFLE */</codeph> method if any
              partition key column in the source table, mentioned in the <codeph>SELECT</codeph>
              clause, does not have column statistics. In this case, use the <codeph>/*
              +NOSHUFFLE */</codeph> hint if you want to override this default behavior.
            </p>
          </li>

          <li>
            If column statistics are available for all partition key columns in the source table
            mentioned in the <codeph>INSERT ... SELECT</codeph> or <codeph>CTAS</codeph> query,
            Impala chooses whether to use the <codeph>/* +SHUFFLE */</codeph> or <codeph>/*
            +NOSHUFFLE */</codeph> technique based on the estimated number of distinct values in
            those columns and the number of nodes involved in the operation. In this case, you
            might need the <codeph>/* +SHUFFLE */</codeph> or the <codeph>/* +NOSHUFFLE
            */</codeph> hint to override the execution plan selected by Impala.
          </li>
        </ul>
      </li>

      <li>
        <codeph>/* +CLUSTERED */</codeph> and <codeph>/* +NOCLUSTERED */</codeph> Hints
        <ul>
          <li>
            <codeph>/* +CLUSTERED */</codeph> sorts data by the partition columns before
            inserting to ensure that only one partition is written at a time per node. Use this
            hint to reduce the number of files kept open and the number of buffers kept in
            memory simultaneously. This technique is primarily useful for inserts into Parquet
            tables, where the large block size requires substantial memory to buffer data for
            multiple output files at once. This hint is available in
            <keyword keyref="impala28_full"/> or higher.
            <p>
              Starting in <keyword keyref="impala30_full"/>, <codeph>/* +CLUSTERED */</codeph>
              is the default behavior for HDFS tables.
            </p>
          </li>

          <li>
            <codeph>/* +NOCLUSTERED */</codeph> does not sort by primary key before insert. This
            hint is available in <keyword
              keyref="impala28_full"/> or higher.
            <p>
              Use this hint when inserting to Kudu tables.
            </p>

            <p>
              In the versions lower than <keyword keyref="impala30_full"/>, <codeph>/*
              +NOCLUSTERED */</codeph> is the default in HDFS tables.
            </p>
          </li>
        </ul>
      </li>
    </ul>
    <p><b>Kudu consideration:</b></p>
    <p conref="../shared/impala_common.xml#common/kudu_hints"/>

    <p rev="IMPALA-2924">
      <b>Hints for scheduling of HDFS blocks:</b>
    </p>

    <p rev="IMPALA-2924">
      The hints <codeph>/* +SCHEDULE_CACHE_LOCAL */</codeph>, <codeph>/* +SCHEDULE_DISK_LOCAL
      */</codeph>, and <codeph>/* +SCHEDULE_REMOTE */</codeph> have the same effect as
      specifying the <codeph>REPLICA_PREFERENCE</codeph> query option with the respective option
      settings of <codeph>CACHE_LOCAL</codeph>, <codeph>DISK_LOCAL</codeph>, or
      <codeph>REMOTE</codeph>. The hint <codeph>/* +RANDOM_REPLICA */</codeph> is the same as
      enabling the <codeph>SCHEDULE_RANDOM_REPLICA</codeph> query option.
    </p>

    <p rev="IMPALA-2924">
      You can use these hints in combination by separating them with commas, for example,
      <codeph>/* +SCHEDULE_CACHE_LOCAL,RANDOM_REPLICA */</codeph>. See
      <xref keyref="replica_preference"/> and <xref keyref="schedule_random_replica"/> for
      information about how these settings influence the way Impala processes HDFS data blocks.
    </p>

    <p rev="IMPALA-2924">
      Specifying the replica preference as a query hint always overrides the query option
      setting. Specifying either the <codeph>SCHEDULE_RANDOM_REPLICA</codeph> query option or
      the corresponding <codeph>RANDOM_REPLICA</codeph> query hint enables the random
      tie-breaking behavior when processing data blocks during the query.
    </p>

    <p>
      <b>Suggestions versus directives:</b>
    </p>

    <p>
      In early Impala releases, hints were always obeyed and so acted more like directives. Once
      Impala gained join order optimizations, sometimes join queries were automatically
      reordered in a way that made a hint irrelevant. Therefore, the hints act more like
      suggestions in Impala 1.2.2 and higher.
    </p>

    <p>
      To force Impala to follow the hinted execution mechanism for a join query, include the
      <codeph>STRAIGHT_JOIN</codeph> keyword in the <codeph>SELECT</codeph> statement. See
      <xref href="impala_perf_joins.xml#straight_join"/> for details. When you use this
      technique, Impala does not reorder the joined tables at all, so you must be careful to
      arrange the join order to put the largest table (or subquery result set) first, then the
      smallest, second smallest, third smallest, and so on. This ordering lets Impala do the
      most I/O-intensive parts of the query using local reads on the DataNodes, and then reduce
      the size of the intermediate result set as much as possible as each subsequent table or
      subquery result set is joined.
    </p>

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

    <p>
      Queries that include subqueries in the <codeph>WHERE</codeph> clause can be rewritten
      internally as join queries. Currently, you cannot apply hints to the joins produced by
      these types of queries.
    </p>

    <p>
      Because hints can prevent queries from taking advantage of new metadata or improvements in
      query planning, use them only when required to work around performance issues, and be
      prepared to remove them when they are no longer required, such as after a new Impala
      release or bug fix.
    </p>

    <p>
      In particular, the <codeph>/* +BROADCAST */</codeph> and <codeph>/* +SHUFFLE */</codeph>
      hints are expected to be needed much less frequently in Impala 1.2.2 and higher, because
      the join order optimization feature in combination with the <codeph>COMPUTE STATS</codeph>
      statement now automatically choose join order and join mechanism without the need to
      rewrite the query and add hints. See <xref href="impala_perf_joins.xml#perf_joins"/> for
      details.
    </p>

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

    <p rev="2.0.0">
      The hints embedded within <codeph>--</codeph> comments are compatible with Hive queries.
      The hints embedded within <codeph>/* */</codeph> comments or <codeph>[ ]</codeph> square
      brackets are not recognized by or not compatible with Hive. For example, Hive raises an
      error for Impala hints within <codeph>/* */</codeph> comments because it does not
      recognize the Impala hint names.
    </p>

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

    <p rev="2.0.0">
      If you use a hint in the query that defines a view, the hint is preserved when you query
      the view. Impala internally rewrites all hints in views to use the <codeph>--</codeph>
      comment notation, so that Hive can query such views without errors due to unrecognized
      hint names.
    </p>

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

    <p>
      For example, this query joins a large customer table with a small lookup table of less
      than 100 rows. The right-hand table can be broadcast efficiently to all nodes involved in
      the join. Thus, you would use the <codeph>/* +broadcast */</codeph> hint to force a
      broadcast join strategy:
    </p>

<codeblock>select straight_join customer.address, state_lookup.state_name
  from customer join <b>/* +broadcast */</b> state_lookup
  on customer.state_id = state_lookup.state_id;</codeblock>

    <p>
      This query joins two large tables of unpredictable size. You might benchmark the query
      with both kinds of hints and find that it is more efficient to transmit portions of each
      table to other nodes for processing. Thus, you would use the <codeph>/* +shuffle
      */</codeph> hint to force a partitioned join strategy:
    </p>

<codeblock>select straight_join weather.wind_velocity, geospatial.altitude
  from weather join <b>/* +shuffle */</b> geospatial
  on weather.lat = geospatial.lat and weather.long = geospatial.long;</codeblock>

    <p>
      For joins involving three or more tables, the hint applies to the tables on either side of
      that specific <codeph>JOIN</codeph> keyword. The <codeph>STRAIGHT_JOIN</codeph> keyword
      ensures that joins are processed in a predictable order from left to right. For example,
      this query joins <codeph>t1</codeph> and <codeph>t2</codeph> using a partitioned join,
      then joins that result set to <codeph>t3</codeph> using a broadcast join:
    </p>

<codeblock>select straight_join t1.name, t2.id, t3.price
  from t1 join <b>/* +shuffle */</b> t2 join <b>/* +broadcast */</b> t3
  on t1.id = t2.id and t2.id = t3.id;</codeblock>

<!-- To do: This is a good place to add more sample output showing before and after EXPLAIN plans. -->

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

    <p>
      For more background information about join queries, see
      <xref href="impala_joins.xml#joins"/>. For performance considerations, see
      <xref href="impala_perf_joins.xml#perf_joins"/>.
    </p>

  </conbody>

</concept>