Google Git
Sign in
apache / impala / 8a582472527eb7300cf5e855d401a90e662de556 / . / docs / build / asf-site-html / topics / impala_replica_preference.html
blob: 0bc8e0c58e8155b9d21a3b84e4d688d28862aef9 [file] [log] [blame]
<!DOCTYPE html
SYSTEM "about:legacy-compat">
<html lang="en"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta charset="UTF-8"><meta name="copyright" content="(C) Copyright 2023"><meta name="DC.rights.owner" content="(C) Copyright 2023"><meta name="DC.Type" content="concept"><meta name="DC.Relation" scheme="URI" content="../topics/impala_query_options.html"><meta name="prodname" content="Impala"><meta name="prodname" content="Impala"><meta name="version" content="Impala 3.4.x"><meta name="version" content="Impala 3.4.x"><meta name="DC.Format" content="XHTML"><meta name="DC.Identifier" content="replica_preference"><link rel="stylesheet" type="text/css" href="../css/commonltr.css"><link rel="stylesheet" type="text/css" href="../css/dita-ot-doc.css"><title>REPLICA_PREFERENCE Query Option (Impala 2.7 or higher only)</title></head><body id="replica_preference"><header role="banner"><!--
The DITA Open Toolkit is licensed for use under the the Apache
Software Foundation License v2.0.
A copy of the Apache Software Foundation License 2.0 is
available at http://opensource.org/licenses/apache2.0.php
This statement must be included in any copies of DITA Open
Toolkit code.
--><div class="header">
<p>Apache Impala</p>
<hr>
</div></header><nav role="toc"><ul><li><a href="../topics/impala_intro.html">Introducing Apache Impala</a></li><li><a href="../topics/impala_concepts.html">Concepts and Architecture</a></li><li><a href="../topics/impala_planning.html">Deployment Planning</a></li><li><a href="../topics/impala_install.html">Installing Impala</a></li><li><a href="../topics/impala_config.html">Managing Impala</a></li><li><a href="../topics/impala_upgrading.html">Upgrading Impala</a></li><li><a href="../topics/impala_processes.html">Starting Impala</a></li><li><a href="../topics/impala_tutorial.html">Tutorials</a></li><li><a href="../topics/impala_admin.html">Administration</a></li><li><a href="../topics/impala_security.html">Impala Security</a></li><li><a href="../topics/impala_langref.html">SQL Reference</a><ul><li><a href="../topics/impala_comments.html">Comments</a></li><li><a href="../topics/impala_datatypes.html">Data Types</a></li><li><a href="../topics/impala_literals.html">Literals</a></li><li><a href="../topics/impala_operators.html">SQL Operators</a></li><li><a href="../topics/impala_schema_objects.html">Schema Objects and Object Names</a></li><li><a href="../topics/impala_transactions.html">Transactions</a></li><li><a href="../topics/impala_langref_sql.html">SQL Statements</a><ul><li><a href="../topics/impala_ddl.html">DDL Statements</a></li><li><a href="../topics/impala_dml.html">DML Statements</a></li><li><a href="../topics/impala_alter_database.html">ALTER DATABASE</a></li><li><a href="../topics/impala_alter_table.html">ALTER TABLE</a></li><li><a href="../topics/impala_alter_view.html">ALTER VIEW</a></li><li><a href="../topics/impala_comment.html">COMMENT</a></li><li><a href="../topics/impala_compute_stats.html">COMPUTE STATS</a></li><li><a href="../topics/impala_create_database.html">CREATE DATABASE</a></li><li><a href="../topics/impala_create_function.html">CREATE FUNCTION</a></li><li><a href="../topics/impala_create_role.html">CREATE ROLE</a></li><li><a href="../topics/impala_create_table.html">CREATE TABLE</a></li><li><a href="../topics/impala_create_view.html">CREATE VIEW</a></li><li><a href="../topics/impala_delete.html">DELETE</a></li><li><a href="../topics/impala_describe.html">DESCRIBE</a></li><li><a href="../topics/impala_drop_database.html">DROP DATABASE</a></li><li><a href="../topics/impala_drop_function.html">DROP FUNCTION</a></li><li><a href="../topics/impala_drop_role.html">DROP ROLE</a></li><li><a href="../topics/impala_drop_stats.html">DROP STATS</a></li><li><a href="../topics/impala_drop_table.html">DROP TABLE</a></li><li><a href="../topics/impala_drop_view.html">DROP VIEW</a></li><li><a href="../topics/impala_explain.html">EXPLAIN</a></li><li><a href="../topics/impala_grant.html">GRANT</a></li><li><a href="../topics/impala_insert.html">INSERT</a></li><li><a href="../topics/impala_invalidate_metadata.html">INVALIDATE METADATA</a></li><li><a href="../topics/impala_load_data.html">LOAD DATA</a></li><li><a href="../topics/impala_refresh.html">REFRESH</a></li><li><a href="../topics/impala_refresh_authorization.html">REFRESH AUTHORIZATION</a></li><li><a href="../topics/impala_refresh_functions.html">REFRESH FUNCTIONS</a></li><li><a href="../topics/impala_revoke.html">REVOKE</a></li><li><a href="../topics/impala_select.html">SELECT</a></li><li><a href="../topics/impala_set.html">SET</a><ul><li><a href="../topics/impala_query_options.html">Query Options for the SET Statement</a><ul><li><a href="../topics/impala_abort_on_error.html">ABORT_ON_ERROR</a></li><li><a href="../topics/impala_allow_erasure_coded_files.html">ALLOW_ERASURE_CODED_FILES</a></li><li><a href="../topics/impala_allow_unsupported_formats.html">ALLOW_UNSUPPORTED_FORMATS</a></li><li><a href="../topics/impala_appx_count_distinct.html">APPX_COUNT_DISTINCT</a></li><li><a href="../topics/impala_batch_size.html">BATCH_SIZE</a></li><li><a href="../topics/impala_broadcast_bytes_limit.html">BROADCAST_BYTES_LIMIT</a></li><li><a href="../topics/impala_buffer_pool_limit.html">BUFFER_POOL_LIMIT</a></li><li><a href="../topics/impala_compression_codec.html">COMPRESSION_CODEC</a></li><li><a href="../topics/impala_compute_stats_min_sample_size.html">COMPUTE_STATS_MIN_SAMPLE_SIZE</a></li><li><a href="../topics/impala_debug_action.html">DEBUG_ACTION</a></li><li><a href="../topics/impala_decimal_v2.html">DECIMAL_V2</a></li><li><a href="../topics/impala_default_file_format.html">DEFAULT_FILE_FORMAT</a></li><li><a href="../topics/impala_default_hints_insert_statement.html">DEFAULT_HINTS_INSERT_STATEMENT</a></li><li><a href="../topics/impala_default_join_distribution_mode.html">DEFAULT_JOIN_DISTRIBUTION_MODE</a></li><li><a href="../topics/impala_default_spillable_buffer_size.html">DEFAULT_SPILLABLE_BUFFER_SIZE</a></li><li><a href="../topics/impala_default_transactional_type.html">DEFAULT_TRANSACTIONAL_TYPE</a></li><li><a href="../topics/impala_delete_stats_in_truncate.html">DELETE_STATS_IN_TRUNCATE</a></li><li><a href="../topics/impala_disable_codegen.html">DISABLE_CODEGEN</a></li><li><a href="../topics/impala_disable_codegen_rows_threshold.html">DISABLE_CODEGEN_ROWS_THRESHOLD</a></li><li><a href="../topics/impala_disable_hbase_num_rows_estimate.html">DISABLE_HBASE_NUM_ROWS_ESTIMATE</a></li><li><a href="../topics/impala_disable_row_runtime_filtering.html">DISABLE_ROW_RUNTIME_FILTERING</a></li><li><a href="../topics/impala_disable_streaming_preaggregations.html">DISABLE_STREAMING_PREAGGREGATIONS</a></li><li><a href="../topics/impala_disable_unsafe_spills.html">DISABLE_UNSAFE_SPILLS</a></li><li><a href="../topics/impala_enable_expr_rewrites.html">ENABLE_EXPR_REWRITES</a></li><li><a href="../topics/impala_exec_single_node_rows_threshold.html">EXEC_SINGLE_NODE_ROWS_THRESHOLD</a></li><li><a href="../topics/impala_exec_time_limit_s.html">EXEC_TIME_LIMIT_S</a></li><li><a href="../topics/impala_explain_level.html">EXPLAIN_LEVEL</a></li><li><a href="../topics/impala_fetch_rows_timeout_ms.html">FETCH_ROWS_TIMEOUT_MS</a></li><li><a href="../topics/impala_join_rows_produced_limit.html">JOIN_ROWS_PRODUCED_LIMIT</a></li><li><a href="../topics/impala_hbase_cache_blocks.html">HBASE_CACHE_BLOCKS</a></li><li><a href="../topics/impala_hbase_caching.html">HBASE_CACHING</a></li><li><a href="../topics/impala_idle_session_timeout.html">IDLE_SESSION_TIMEOUT</a></li><li><a href="../topics/impala_kudu_read_mode.html">KUDU_READ_MODE</a></li><li><a href="../topics/impala_live_progress.html">LIVE_PROGRESS</a></li><li><a href="../topics/impala_live_summary.html">LIVE_SUMMARY</a></li><li><a href="../topics/impala_max_errors.html">MAX_ERRORS</a></li><li><a href="../topics/impala_max_mem_estimate_for_admission.html">MAX_MEM_ESTIMATE_FOR_ADMISSION</a></li><li><a href="../topics/impala_max_result_spooling_mem.html">MAX_RESULT_SPOOLING_MEM</a></li><li><a href="../topics/impala_max_row_size.html">MAX_ROW_SIZE</a></li><li><a href="../topics/impala_max_scan_range_length.html">MAX_SCAN_RANGE_LENGTH</a></li><li><a href="../topics/impala_max_spilled_result_spooling_mem.html">MAX_SPILLED_RESULT_SPOOLING_MEM</a></li><li><a href="../topics/impala_mem_limit.html">MEM_LIMIT</a></li><li><a href="../topics/impala_min_spillable_buffer_size.html">MIN_SPILLABLE_BUFFER_SIZE</a></li><li><a href="../topics/impala_mt_dop.html">MT_DOP</a></li><li><a href="../topics/impala_num_nodes.html">NUM_NODES</a></li><li><a href="../topics/impala_num_rows_produced_limit.html">NUM_ROWS_PRODUCED_LIMIT</a></li><li><a href="../topics/impala_num_scanner_threads.html">NUM_SCANNER_THREADS</a></li><li><a href="../topics/impala_optimize_partition_key_scans.html">OPTIMIZE_PARTITION_KEY_SCANS</a></li><li><a href="../topics/impala_parquet_compression_codec.html">PARQUET_COMPRESSION_CODEC</a></li><li><a href="../topics/impala_parquet_annotate_strings_utf8.html">PARQUET_ANNOTATE_STRINGS_UTF8</a></li><li><a href="../topics/impala_parquet_array_resolution.html">PARQUET_ARRAY_RESOLUTION</a></li><li><a href="../topics/impala_parquet_dictionary_filtering.html">PARQUET_DICTIONARY_FILTERING</a></li><li><a href="../topics/impala_parquet_fallback_schema_resolution.html">PARQUET_FALLBACK_SCHEMA_RESOLUTION</a></li><li><a href="../topics/impala_parquet_file_size.html">PARQUET_FILE_SIZE</a></li><li><a href="../topics/impala_parquet_object_store_split_size.html">PARQUET_OBJECT_STORE_SPLIT_SIZE</a></li><li><a href="../topics/impala_parquet_page_row_count_limit.html">PARQUET_PAGE_ROW_COUNT_LIMIT</a></li><li><a href="../topics/impala_parquet_read_statistics.html">PARQUET_READ_STATISTICS</a></li><li><a href="../topics/impala_parquet_read_page_index.html">PARQUET_READ_PAGE_INDEX</a></li><li><a href="../topics/impala_parquet_write_page_index.html">PARQUET_WRITE_PAGE_INDEX</a></li><li><a href="../topics/impala_prefetch_mode.html">PREFETCH_MODE</a></li><li><a href="../topics/impala_query_timeout_s.html">QUERY_TIMEOUT_S</a></li><li><a href="../topics/impala_refresh_updated_hms.html">REFRESH_UPDATED_HMS_PARTITIONS</a></li><li class="active"><a href="../topics/impala_replica_preference.html">REPLICA_PREFERENCE</a></li><li><a href="../topics/impala_request_pool.html">REQUEST_POOL</a></li><li><a href="../topics/impala_resource_trace_ratio.html">RESOURCE_TRACE_RATIO</a></li><li><a href="../topics/impala_retry_failed_queries.html">RETRY_FAILED_QUERIES</a></li><li><a href="../topics/impala_runtime_bloom_filter_size.html">RUNTIME_BLOOM_FILTER_SIZE</a></li><li><a href="../topics/impala_runtime_filter_max_size.html">RUNTIME_FILTER_MAX_SIZE</a></li><li><a href="../topics/impala_runtime_filter_min_size.html">RUNTIME_FILTER_MIN_SIZE</a></li><li><a href="../topics/impala_runtime_filter_mode.html">RUNTIME_FILTER_MODE</a></li><li><a href="../topics/impala_runtime_filter_wait_time_ms.html">RUNTIME_FILTER_WAIT_TIME_MS</a></li><li><a href="../topics/impala_s3_skip_insert_staging.html">S3_SKIP_INSERT_STAGING</a></li><li><a href="../topics/impala_scan_bytes_limit.html">SCAN_BYTES_LIMIT</a></li><li><a href="../topics/impala_schedule_random_replica.html">SCHEDULE_RANDOM_REPLICA</a></li><li><a href="../topics/impala_scratch_limit.html">SCRATCH_LIMIT</a></li><li><a href="../topics/impala_shuffle_distinct_exprs.html">SHUFFLE_DISTINCT_EXPRS</a></li><li><a href="../topics/impala_spool_query_results.html">SPOOL_QUERY_RESULTS</a></li><li><a href="../topics/impala_support_start_over.html">SUPPORT_START_OVER</a></li><li><a href="../topics/impala_sync_ddl.html">SYNC_DDL</a></li><li><a href="../topics/impala_thread_reservation_aggregate_limit.html">THREAD_RESERVATION_AGGREGATE_LIMIT</a></li><li><a href="../topics/impala_thread_reservation_limit.html">THREAD_RESERVATION_LIMIT</a></li><li><a href="../topics/impala_timezone.html">TIMEZONE</a></li><li><a href="../topics/impala_topn_bytes_limit.html">TOPN_BYTES_LIMIT</a></li><li><a href="../topics/impala_utf8_mode.html">UTF8_MODE</a></li><li><a href="../topics/impala_expand_complex_types.html">EXPAND_COMPLEX_TYPES</a></li></ul></li></ul></li><li><a href="../topics/impala_show.html">SHOW</a></li><li><a href="../topics/impala_shutdown.html">SHUTDOWN</a></li><li><a href="../topics/impala_truncate_table.html">TRUNCATE TABLE</a></li><li><a href="../topics/impala_update.html">UPDATE</a></li><li><a href="../topics/impala_upsert.html">UPSERT</a></li><li><a href="../topics/impala_use.html">USE</a></li><li><a href="../topics/impala_values.html">VALUES</a></li><li><a href="../topics/impala_hints.html">Optimizer Hints</a></li></ul></li><li><a href="../topics/impala_functions.html">Built-In Functions</a></li><li><a href="../topics/impala_udf.html">User-Defined Functions (UDFs)</a></li><li><a href="../topics/impala_langref_unsupported.html">SQL Differences Between Impala and Hive</a></li><li><a href="../topics/impala_porting.html">Porting SQL</a></li><li><a href="../topics/impala_utf_8.html">UTF-8 Support</a></li></ul></li><li><a href="../topics/impala_performance.html">Performance Tuning</a></li><li><a href="../topics/impala_scalability.html">Scalability Considerations</a></li><li><a href="../topics/impala_resource_management.html">Resource Management</a></li><li><a href="../topics/impala_partitioning.html">Partitioning</a></li><li><a href="../topics/impala_file_formats.html">File Formats</a></li><li><a href="../topics/impala_kudu.html">Using Impala to Query Kudu Tables</a></li><li><a href="../topics/impala_hbase.html">HBase Tables</a></li><li><a href="../topics/impala_iceberg.html">Iceberg Tables</a></li><li><a href="../topics/impala_s3.html">S3 Tables</a></li><li><a href="../topics/impala_adls.html">ADLS Tables</a></li><li><a href="../topics/impala_isilon.html">Isilon Storage</a></li><li><a href="../topics/impala_ozone.html">Ozone Storage</a></li><li><a href="../topics/impala_logging.html">Logging</a></li><li><a href="../topics/impala_client.html">Client Access</a></li><li><a href="../topics/impala_fault_tolerance.html">Fault Tolerance</a></li><li><a href="../topics/impala_troubleshooting.html">Troubleshooting Impala</a></li><li><a href="../topics/impala_ports.html">Ports Used by Impala</a></li><li><a href="../topics/impala_reserved_words.html">Impala Reserved Words</a></li><li><a href="../topics/impala_faq.html">Impala Frequently Asked Questions</a></li><li><a href="../topics/impala_release_notes.html">Impala Release Notes</a></li></ul></nav><main role="main"><article role="article" aria-labelledby="ariaid-title1">
<h1 class="title topictitle1" id="ariaid-title1">REPLICA_PREFERENCE Query Option (<span class="keyword">Impala 2.7</span> or higher only)</h1>
<div class="body conbody">
<p class="p">
</p>
<p class="p">
The <code class="ph codeph">REPLICA_PREFERENCE</code> query option lets you distribute the work more
evenly if hotspots and bottlenecks persist. It causes the access cost of all replicas of a
data block to be considered equal to or worse than the configured value. This allows
Impala to schedule reads to suboptimal replicas (e.g. local in the presence of cached
ones) in order to distribute the work across more executor nodes.
</p>
<p class="p">
Allowed values are: <code class="ph codeph">CACHE_LOCAL</code> (<code class="ph codeph">0</code>),
<code class="ph codeph">DISK_LOCAL</code> (<code class="ph codeph">2</code>), <code class="ph codeph">REMOTE</code>
(<code class="ph codeph">4</code>)
</p>
<p class="p">
<strong class="ph b">Type:</strong> Enum
</p>
<p class="p">
<strong class="ph b">Default:</strong> <code class="ph codeph">CACHE_LOCAL (0)</code>
</p>
<p class="p">
<strong class="ph b">Added in:</strong> <span class="keyword">Impala 2.7.0</span>
</p>
<p class="p">
<strong class="ph b">Usage Notes:</strong>
</p>
<p class="p">
By default Impala selects the best replica it can find in terms of access cost. The
preferred order is cached, local, and remote. With <code class="ph codeph">REPLICA_PREFERENCE</code>,
the preference of all replicas are capped at the selected value. For example, when
<code class="ph codeph">REPLICA_PREFERENCE</code> is set to <code class="ph codeph">DISK_LOCAL</code>, cached and
local replicas are treated with the equal preference. When set to
<code class="ph codeph">REMOTE</code>, all three types of replicas, cached, local, remote, are treated
with equal preference.
</p>
<p class="p">
<strong class="ph b">Related information:</strong>
</p>
<p class="p">
<a class="xref" href="impala_perf_hdfs_caching.html#hdfs_caching">Using HDFS Caching with Impala (Impala 2.1 or higher only)</a>,
<a class="xref" href="impala_schedule_random_replica.html#schedule_random_replica">SCHEDULE_RANDOM_REPLICA Query Option (Impala 2.5 or higher only)</a>
</p>
</div>
<nav role="navigation" class="related-links"><div class="familylinks"><div class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_query_options.html">Query Options for the SET Statement</a></div></div></nav></article></main></body></html>