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Explain how doing COMPUTE INCREMENTAL STATS for the first time starts over and discards any previous stats from COMPUTE STATS. As a consequence, moved some wording and examples into impala_common.xml so that content could be used in multiple places. Also made a new subtopic on the "Partitioning" page because I saw COMPUTE INCREMENTAL STATS wasn't mentioned there. Change-Id: Ia53a6518ce5541e5c9a2cd896856ce042a599b03 Reviewed-on: http://gerrit.cloudera.org:8080/7999 Reviewed-by: Alex Behm <alex.behm@cloudera.com> Tested-by: Impala Public Jenkins
451 lines
22 KiB
XML
451 lines
22 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept rev="1.2.2" id="compute_stats">
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<title>COMPUTE STATS Statement</title>
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<titlealts audience="PDF"><navtitle>COMPUTE STATS</navtitle></titlealts>
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<prolog>
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<metadata>
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<data name="Category" value="Impala"/>
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<data name="Category" value="Performance"/>
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<data name="Category" value="Scalability"/>
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<data name="Category" value="ETL"/>
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<data name="Category" value="Ingest"/>
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<data name="Category" value="SQL"/>
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<data name="Category" value="Tables"/>
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<data name="Category" value="Developers"/>
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<data name="Category" value="Data Analysts"/>
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</metadata>
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</prolog>
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<conbody>
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<p>
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<indexterm audience="hidden">COMPUTE STATS statement</indexterm>
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Gathers information about volume and distribution of data in a table and all associated columns and
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partitions. The information is stored in the metastore database, and used by Impala to help optimize queries.
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For example, if Impala can determine that a table is large or small, or has many or few distinct values it
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can organize parallelize the work appropriately for a join query or insert operation. For details about the
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kinds of information gathered by this statement, see <xref href="impala_perf_stats.xml#perf_stats"/>.
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</p>
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<p conref="../shared/impala_common.xml#common/syntax_blurb"/>
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<codeblock rev="2.1.0">COMPUTE STATS [<varname>db_name</varname>.]<varname>table_name</varname>
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COMPUTE INCREMENTAL STATS [<varname>db_name</varname>.]<varname>table_name</varname> [PARTITION (<varname>partition_spec</varname>)]
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<varname>partition_spec</varname> ::= <varname>simple_partition_spec</varname> | <ph rev="IMPALA-1654"><varname>complex_partition_spec</varname></ph>
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<varname>simple_partition_spec</varname> ::= <varname>partition_col</varname>=<varname>constant_value</varname>
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<ph rev="IMPALA-1654"><varname>complex_partition_spec</varname> ::= <varname>comparison_expression_on_partition_col</varname></ph>
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</codeblock>
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<p conref="../shared/impala_common.xml#common/incremental_partition_spec"/>
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<p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>
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<p>
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Originally, Impala relied on users to run the Hive <codeph>ANALYZE TABLE</codeph> statement, but that method
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of gathering statistics proved unreliable and difficult to use. The Impala <codeph>COMPUTE STATS</codeph>
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statement is built from the ground up to improve the reliability and user-friendliness of this operation.
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<codeph>COMPUTE STATS</codeph> does not require any setup steps or special configuration. You only run a
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single Impala <codeph>COMPUTE STATS</codeph> statement to gather both table and column statistics, rather
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than separate Hive <codeph>ANALYZE TABLE</codeph> statements for each kind of statistics.
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</p>
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<p rev="2.1.0">
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The <codeph>COMPUTE INCREMENTAL STATS</codeph> variation is a shortcut for partitioned tables that works on a
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subset of partitions rather than the entire table. The incremental nature makes it suitable for large tables
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with many partitions, where a full <codeph>COMPUTE STATS</codeph> operation takes too long to be practical
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each time a partition is added or dropped. See <xref href="impala_perf_stats.xml#perf_stats_incremental"/>
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for full usage details.
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</p>
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<note type="important">
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<p conref="../shared/impala_common.xml#common/cs_or_cis"/>
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<p conref="../shared/impala_common.xml#common/incremental_stats_after_full"/>
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<p conref="../shared/impala_common.xml#common/incremental_stats_caveats"/>
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</note>
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<p>
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<codeph>COMPUTE INCREMENTAL STATS</codeph> only applies to partitioned tables. If you use the
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<codeph>INCREMENTAL</codeph> clause for an unpartitioned table, Impala automatically uses the original
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<codeph>COMPUTE STATS</codeph> statement. Such tables display <codeph>false</codeph> under the
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<codeph>Incremental stats</codeph> column of the <codeph>SHOW TABLE STATS</codeph> output.
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</p>
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<note>
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Because many of the most performance-critical and resource-intensive operations rely on table and column
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statistics to construct accurate and efficient plans, <codeph>COMPUTE STATS</codeph> is an important step at
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the end of your ETL process. Run <codeph>COMPUTE STATS</codeph> on all tables as your first step during
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performance tuning for slow queries, or troubleshooting for out-of-memory conditions:
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<ul>
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<li>
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Accurate statistics help Impala construct an efficient query plan for join queries, improving performance
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and reducing memory usage.
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</li>
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<li>
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Accurate statistics help Impala distribute the work effectively for insert operations into Parquet
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tables, improving performance and reducing memory usage.
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</li>
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<li rev="1.3.0">
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Accurate statistics help Impala estimate the memory required for each query, which is important when you
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use resource management features, such as admission control and the YARN resource management framework.
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The statistics help Impala to achieve high concurrency, full utilization of available memory, and avoid
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contention with workloads from other Hadoop components.
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</li>
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<li rev="IMPALA-4572">
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In <keyword keyref="impala28_full"/> and higher, when you run the
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<codeph>COMPUTE STATS</codeph> or <codeph>COMPUTE INCREMENTAL STATS</codeph>
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statement against a Parquet table, Impala automatically applies the query
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option setting <codeph>MT_DOP=4</codeph> to increase the amount of intra-node
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parallelism during this CPU-intensive operation. See <xref keyref="mt_dop"/>
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for details about what this query option does and how to use it with
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CPU-intensive <codeph>SELECT</codeph> statements.
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</li>
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</ul>
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</note>
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<p rev="IMPALA-1654">
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<b>Computing stats for groups of partitions:</b>
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</p>
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<p rev="IMPALA-1654">
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In <keyword keyref="impala28_full"/> and higher, you can run <codeph>COMPUTE INCREMENTAL STATS</codeph>
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on multiple partitions, instead of the entire table or one partition at a time. You include
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comparison operators other than <codeph>=</codeph> in the <codeph>PARTITION</codeph> clause,
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and the <codeph>COMPUTE INCREMENTAL STATS</codeph> statement applies to all partitions that
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match the comparison expression.
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</p>
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<p rev="IMPALA-1654">
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For example, the <codeph>INT_PARTITIONS</codeph> table contains 4 partitions.
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The following <codeph>COMPUTE INCREMENTAL STATS</codeph> statements affect some but not all
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partitions, as indicated by the <codeph>Updated <varname>n</varname> partition(s)</codeph>
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messages. The partitions that are affected depend on values in the partition key column <codeph>X</codeph>
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that match the comparison expression in the <codeph>PARTITION</codeph> clause.
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</p>
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<codeblock rev="IMPALA-1654"><![CDATA[
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show partitions int_partitions;
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+-------+-------+--------+------+--------------+-------------------+---------+...
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| x | #Rows | #Files | Size | Bytes Cached | Cache Replication | Format |...
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+-------+-------+--------+------+--------------+-------------------+---------+...
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| 99 | -1 | 0 | 0B | NOT CACHED | NOT CACHED | PARQUET |...
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| 120 | -1 | 0 | 0B | NOT CACHED | NOT CACHED | TEXT |...
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| 150 | -1 | 0 | 0B | NOT CACHED | NOT CACHED | TEXT |...
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| 200 | -1 | 0 | 0B | NOT CACHED | NOT CACHED | TEXT |...
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| Total | -1 | 0 | 0B | 0B | | |...
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+-------+-------+--------+------+--------------+-------------------+---------+...
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compute incremental stats int_partitions partition (x < 100);
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+-----------------------------------------+
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| summary |
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+-----------------------------------------+
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| Updated 1 partition(s) and 1 column(s). |
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+-----------------------------------------+
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compute incremental stats int_partitions partition (x in (100, 150, 200));
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+-----------------------------------------+
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| summary |
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+-----------------------------------------+
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| Updated 2 partition(s) and 1 column(s). |
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+-----------------------------------------+
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compute incremental stats int_partitions partition (x between 100 and 175);
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+-----------------------------------------+
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| summary |
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+-----------------------------------------+
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| Updated 2 partition(s) and 1 column(s). |
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+-----------------------------------------+
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compute incremental stats int_partitions partition (x in (100, 150, 200) or x < 100);
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+-----------------------------------------+
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| summary |
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+-----------------------------------------+
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| Updated 3 partition(s) and 1 column(s). |
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+-----------------------------------------+
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compute incremental stats int_partitions partition (x != 150);
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+-----------------------------------------+
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| summary |
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+-----------------------------------------+
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| Updated 3 partition(s) and 1 column(s). |
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+-----------------------------------------+
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]]>
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</codeblock>
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<p conref="../shared/impala_common.xml#common/complex_types_blurb"/>
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<p rev="2.3.0">
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Currently, the statistics created by the <codeph>COMPUTE STATS</codeph> statement do not include
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information about complex type columns. The column stats metrics for complex columns are always shown
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as -1. For queries involving complex type columns, Impala uses
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heuristics to estimate the data distribution within such columns.
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</p>
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<p conref="../shared/impala_common.xml#common/hbase_blurb"/>
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<p>
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<codeph>COMPUTE STATS</codeph> works for HBase tables also. The statistics gathered for HBase tables are
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somewhat different than for HDFS-backed tables, but that metadata is still used for optimization when HBase
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tables are involved in join queries.
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</p>
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<p conref="../shared/impala_common.xml#common/s3_blurb"/>
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<p rev="2.2.0">
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<codeph>COMPUTE STATS</codeph> also works for tables where data resides in the Amazon Simple Storage Service (S3).
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See <xref href="impala_s3.xml#s3"/> for details.
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</p>
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<p conref="../shared/impala_common.xml#common/performance_blurb"/>
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<p>
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The statistics collected by <codeph>COMPUTE STATS</codeph> are used to optimize join queries
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<codeph>INSERT</codeph> operations into Parquet tables, and other resource-intensive kinds of SQL statements.
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See <xref href="impala_perf_stats.xml#perf_stats"/> for details.
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</p>
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<p>
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For large tables, the <codeph>COMPUTE STATS</codeph> statement itself might take a long time and you
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might need to tune its performance. The <codeph>COMPUTE STATS</codeph> statement does not work with the
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<codeph>EXPLAIN</codeph> statement, or the <codeph>SUMMARY</codeph> command in <cmdname>impala-shell</cmdname>.
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You can use the <codeph>PROFILE</codeph> statement in <cmdname>impala-shell</cmdname> to examine timing information
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for the statement as a whole. If a basic <codeph>COMPUTE STATS</codeph> statement takes a long time for a
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partitioned table, consider switching to the <codeph>COMPUTE INCREMENTAL STATS</codeph> syntax so that only
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newly added partitions are analyzed each time.
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</p>
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<p conref="../shared/impala_common.xml#common/example_blurb"/>
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<p>
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This example shows two tables, <codeph>T1</codeph> and <codeph>T2</codeph>, with a small number distinct
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values linked by a parent-child relationship between <codeph>T1.ID</codeph> and <codeph>T2.PARENT</codeph>.
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<codeph>T1</codeph> is tiny, while <codeph>T2</codeph> has approximately 100K rows. Initially, the statistics
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includes physical measurements such as the number of files, the total size, and size measurements for
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fixed-length columns such as with the <codeph>INT</codeph> type. Unknown values are represented by -1. After
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running <codeph>COMPUTE STATS</codeph> for each table, much more information is available through the
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<codeph>SHOW STATS</codeph> statements. If you were running a join query involving both of these tables, you
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would need statistics for both tables to get the most effective optimization for the query.
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</p>
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<!-- Note: chopped off any excess characters at position 87 and after,
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to avoid weird wrapping in PDF.
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Applies to any subsequent examples with output from SHOW ... STATS too. -->
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<codeblock>[localhost:21000] > show table stats t1;
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Query: show table stats t1
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+-------+--------+------+--------+
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| #Rows | #Files | Size | Format |
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+-------+--------+------+--------+
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| -1 | 1 | 33B | TEXT |
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+-------+--------+------+--------+
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Returned 1 row(s) in 0.02s
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[localhost:21000] > show table stats t2;
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Query: show table stats t2
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+-------+--------+----------+--------+
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| #Rows | #Files | Size | Format |
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+-------+--------+----------+--------+
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| -1 | 28 | 960.00KB | TEXT |
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+-------+--------+----------+--------+
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Returned 1 row(s) in 0.01s
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[localhost:21000] > show column stats t1;
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Query: show column stats t1
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+--------+--------+------------------+--------+----------+----------+
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| Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
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+--------+--------+------------------+--------+----------+----------+
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| id | INT | -1 | -1 | 4 | 4 |
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| s | STRING | -1 | -1 | -1 | -1 |
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+--------+--------+------------------+--------+----------+----------+
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Returned 2 row(s) in 1.71s
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[localhost:21000] > show column stats t2;
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Query: show column stats t2
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+--------+--------+------------------+--------+----------+----------+
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| Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
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+--------+--------+------------------+--------+----------+----------+
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| parent | INT | -1 | -1 | 4 | 4 |
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| s | STRING | -1 | -1 | -1 | -1 |
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+--------+--------+------------------+--------+----------+----------+
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Returned 2 row(s) in 0.01s
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[localhost:21000] > compute stats t1;
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Query: compute stats t1
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+-----------------------------------------+
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| summary |
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+-----------------------------------------+
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| Updated 1 partition(s) and 2 column(s). |
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+-----------------------------------------+
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Returned 1 row(s) in 5.30s
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[localhost:21000] > show table stats t1;
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Query: show table stats t1
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+-------+--------+------+--------+
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| #Rows | #Files | Size | Format |
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+-------+--------+------+--------+
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| 3 | 1 | 33B | TEXT |
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+-------+--------+------+--------+
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Returned 1 row(s) in 0.01s
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[localhost:21000] > show column stats t1;
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Query: show column stats t1
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+--------+--------+------------------+--------+----------+----------+
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| Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
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+--------+--------+------------------+--------+----------+----------+
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| id | INT | 3 | -1 | 4 | 4 |
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| s | STRING | 3 | -1 | -1 | -1 |
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+--------+--------+------------------+--------+----------+----------+
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Returned 2 row(s) in 0.02s
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[localhost:21000] > compute stats t2;
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Query: compute stats t2
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+-----------------------------------------+
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| summary |
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+-----------------------------------------+
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| Updated 1 partition(s) and 2 column(s). |
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+-----------------------------------------+
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Returned 1 row(s) in 5.70s
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[localhost:21000] > show table stats t2;
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Query: show table stats t2
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+-------+--------+----------+--------+
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| #Rows | #Files | Size | Format |
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+-------+--------+----------+--------+
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| 98304 | 1 | 960.00KB | TEXT |
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+-------+--------+----------+--------+
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Returned 1 row(s) in 0.03s
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[localhost:21000] > show column stats t2;
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Query: show column stats t2
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+--------+--------+------------------+--------+----------+----------+
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| Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
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+--------+--------+------------------+--------+----------+----------+
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| parent | INT | 3 | -1 | 4 | 4 |
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| s | STRING | 6 | -1 | 14 | 9.3 |
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+--------+--------+------------------+--------+----------+----------+
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Returned 2 row(s) in 0.01s</codeblock>
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<p rev="2.1.0">
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The following example shows how to use the <codeph>INCREMENTAL</codeph> clause, available in Impala 2.1.0 and
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higher. The <codeph>COMPUTE INCREMENTAL STATS</codeph> syntax lets you collect statistics for newly added or
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changed partitions, without rescanning the entire table.
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</p>
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<codeblock conref="../shared/impala_common.xml#common/compute_stats_walkthrough"/>
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<p conref="../shared/impala_common.xml#common/file_format_blurb"/>
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<p>
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The <codeph>COMPUTE STATS</codeph> statement works with tables created with any of the file formats supported
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by Impala. See <xref href="impala_file_formats.xml#file_formats"/> for details about working with the
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different file formats. The following considerations apply to <codeph>COMPUTE STATS</codeph> depending on the
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file format of the table.
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</p>
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<p>
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The <codeph>COMPUTE STATS</codeph> statement works with text tables with no restrictions. These tables can be
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created through either Impala or Hive.
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</p>
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<p>
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The <codeph>COMPUTE STATS</codeph> statement works with Parquet tables. These tables can be created through
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either Impala or Hive.
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</p>
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<p>
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The <codeph>COMPUTE STATS</codeph> statement works with Avro tables without restriction in <keyword keyref="impala22_full"/>
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and higher. In earlier releases, <codeph>COMPUTE STATS</codeph> worked only for Avro tables created through Hive,
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and required the <codeph>CREATE TABLE</codeph> statement to use SQL-style column names and types rather than an
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Avro-style schema specification.
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</p>
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<p>
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The <codeph>COMPUTE STATS</codeph> statement works with RCFile tables with no restrictions. These tables can
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be created through either Impala or Hive.
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</p>
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<p>
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The <codeph>COMPUTE STATS</codeph> statement works with SequenceFile tables with no restrictions. These
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tables can be created through either Impala or Hive.
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</p>
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<p>
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The <codeph>COMPUTE STATS</codeph> statement works with partitioned tables, whether all the partitions use
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the same file format, or some partitions are defined through <codeph>ALTER TABLE</codeph> to use different
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file formats.
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</p>
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<p conref="../shared/impala_common.xml#common/ddl_blurb"/>
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<p conref="../shared/impala_common.xml#common/cancel_blurb_maybe"/>
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<p conref="../shared/impala_common.xml#common/restrictions_blurb"/>
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<note conref="../shared/impala_common.xml#common/compute_stats_nulls"/>
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<p conref="../shared/impala_common.xml#common/internals_blurb"/>
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<p>
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Behind the scenes, the <codeph>COMPUTE STATS</codeph> statement
|
|
executes two statements: one to count the rows of each partition
|
|
in the table (or the entire table if unpartitioned) through the
|
|
<codeph>COUNT(*)</codeph> function,
|
|
and another to count the approximate number of distinct values
|
|
in each column through the <codeph>NDV()</codeph> function.
|
|
You might see these queries in your monitoring and diagnostic displays.
|
|
The same factors that affect the performance, scalability, and
|
|
execution of other queries (such as parallel execution, memory usage,
|
|
admission control, and timeouts) also apply to the queries run by the
|
|
<codeph>COMPUTE STATS</codeph> statement.
|
|
</p>
|
|
|
|
<p conref="../shared/impala_common.xml#common/permissions_blurb"/>
|
|
<p rev="">
|
|
The user ID that the <cmdname>impalad</cmdname> daemon runs under,
|
|
typically the <codeph>impala</codeph> user, must have read
|
|
permission for all affected files in the source directory:
|
|
all files in the case of an unpartitioned table or
|
|
a partitioned table in the case of <codeph>COMPUTE STATS</codeph>;
|
|
or all the files in partitions without incremental stats in
|
|
the case of <codeph>COMPUTE INCREMENTAL STATS</codeph>.
|
|
It must also have read and execute permissions for all
|
|
relevant directories holding the data files.
|
|
(Essentially, <codeph>COMPUTE STATS</codeph> requires the
|
|
same permissions as the underlying <codeph>SELECT</codeph> queries it runs
|
|
against the table.)
|
|
</p>
|
|
|
|
<p rev="kudu" conref="../shared/impala_common.xml#common/kudu_blurb"/>
|
|
|
|
<p rev="IMPALA-2830">
|
|
The <codeph>COMPUTE STATS</codeph> statement applies to Kudu tables.
|
|
Impala does not compute the number of rows for each partition for
|
|
Kudu tables. Therefore, you do not need to re-run the operation when
|
|
you see -1 in the <codeph># Rows</codeph> column of the output from
|
|
<codeph>SHOW TABLE STATS</codeph>. That column always shows -1 for
|
|
all Kudu tables.
|
|
</p>
|
|
|
|
<p conref="../shared/impala_common.xml#common/related_info"/>
|
|
|
|
<p>
|
|
<xref href="impala_drop_stats.xml#drop_stats"/>, <xref href="impala_show.xml#show_table_stats"/>,
|
|
<xref href="impala_show.xml#show_column_stats"/>, <xref href="impala_perf_stats.xml#perf_stats"/>
|
|
</p>
|
|
</conbody>
|
|
</concept>
|