Cloudera Enterprise 5.15.x | Other versions

COMPUTE STATS Statement

The COMPUTE STATS statement gathers information about volume and distribution of data in a table and all associated columns and partitions. The information is stored in the metastore database, and used by Impala to help optimize queries. For example, if Impala can determine that a table is large or small, or has many or few distinct values it can organize and parallelize the work appropriately for a join query or insert operation. For details about the kinds of information gathered by this statement, see Table and Column Statistics.

Syntax:

COMPUTE STATS [db_name.]table_name  [ ( column_list ) ] [TABLESAMPLE SYSTEM(percentage) [REPEATABLE(seed)]]

column_list ::= column_name [ , column_name, ... ]

COMPUTE INCREMENTAL STATS [db_name.]table_name [PARTITION (partition_spec)]

partition_spec ::= partition_col=constant_value

partition_spec ::= simple_partition_spec | complex_partition_spec

simple_partition_spec ::= partition_col=constant_value

complex_partition_spec ::= comparison_expression_on_partition_col

The PARTITION clause is only allowed in combination with the INCREMENTAL clause. It is optional for COMPUTE INCREMENTAL STATS, and required for DROP INCREMENTAL STATS. Whenever you specify partitions through the PARTITION (partition_spec) clause in a COMPUTE INCREMENTAL STATS or DROP INCREMENTAL STATS statement, you must include all the partitioning columns in the specification, and specify constant values for all the partition key columns.

Usage notes:

Originally, Impala relied on users to run the Hive ANALYZE TABLE statement, but that method of gathering statistics proved unreliable and difficult to use. The Impala COMPUTE STATS statement was built to improve the reliability and user-friendliness of this operation. COMPUTE STATS does not require any setup steps or special configuration. You only run a single Impala COMPUTE STATS statement to gather both table and column statistics, rather than separate Hive ANALYZE TABLE statements for each kind of statistics.

For non-incremental COMPUTE STATS statement, the columns for which statistics are computed can be specified with an optional comma-separate list of columns.

If no column list is given, the COMPUTE STATS statement computes column-level statistics for all columns of the table. This adds potentially unneeded work for columns whose stats are not needed by queries. It can be especially costly for very wide tables and unneeded large string fields.

COMPUTE STATS returns an error when a specified column cannot be analyzed, such as when the column does not exist, the column is of an unsupported type for COMPUTE STATS, e.g. colums of complex types, or the column is a partitioning column.

If an empty column list is given, no column is analyzed by COMPUTE STATS.

In CDH 5.15 / Impala 2.12 and higher, an optional TABLESAMPLE clause immediately after a table reference specifies that the COMPUTE STATS operation only processes a specified percentage of the table data. For tables that are so large that a full COMPUTE STATS operation is impractical, you can use COMPUTE STATS with a TABLESAMPLE clause to extrapolate statistics from a sample of the table data. See Table and Column Statistics about the experimental stats extrapolation and sampling features.

The COMPUTE INCREMENTAL STATS variation is a shortcut for partitioned tables that works on a subset of partitions rather than the entire table. The incremental nature makes it suitable for large tables with many partitions, where a full COMPUTE STATS operation takes too long to be practical each time a partition is added or dropped. See Table and Column Statistics for full usage details.

Important:

For a particular table, use either COMPUTE STATS or COMPUTE INCREMENTAL STATS, but never combine the two or alternate between them. If you switch from COMPUTE STATS to COMPUTE INCREMENTAL STATS during the lifetime of a table, or vice versa, drop all statistics by running DROP STATS before making the switch.

When you run COMPUTE INCREMENTAL STATS on a table for the first time, the statistics are computed again from scratch regardless of whether the table already has statistics. Therefore, expect a one-time resource-intensive operation for scanning the entire table when running COMPUTE INCREMENTAL STATS for the first time on a given table.

For a table with a huge number of partitions and many columns, the approximately 400 bytes of metadata per column per partition can add up to significant memory overhead, as it must be cached on the catalogd host and on every impalad host that is eligible to be a coordinator. If this metadata for all tables combined exceeds 2 GB, you might experience service downtime.

COMPUTE INCREMENTAL STATS only applies to partitioned tables. If you use the INCREMENTAL clause for an unpartitioned table, Impala automatically uses the original COMPUTE STATS statement. Such tables display false under the Incremental stats column of the SHOW TABLE STATS output.

Note:

Because many of the most performance-critical and resource-intensive operations rely on table and column statistics to construct accurate and efficient plans, COMPUTE STATS is an important step at the end of your ETL process. Run COMPUTE STATS on all tables as your first step during performance tuning for slow queries, or troubleshooting for out-of-memory conditions:

Accurate statistics help Impala construct an efficient query plan for join queries, improving performance and reducing memory usage.
Accurate statistics help Impala distribute the work effectively for insert operations into Parquet tables, improving performance and reducing memory usage.
Accurate statistics help Impala estimate the memory required for each query, which is important when you use resource management features, such as admission control and the YARN resource management framework. The statistics help Impala to achieve high concurrency, full utilization of available memory, and avoid contention with workloads from other Hadoop components.
In CDH 5.10 / Impala 2.8 and higher, when you run the COMPUTE STATS or COMPUTE INCREMENTAL STATS statement against a Parquet table, Impala automatically applies the query option setting MT_DOP=4 to increase the amount of intra-node parallelism during this CPU-intensive operation. See MT_DOP Query Option for details about what this query option does and how to use it with CPU-intensive SELECT statements.

Computing stats for groups of partitions:

In CDH 5.10 / Impala 2.8 and higher, you can run COMPUTE INCREMENTAL STATS on multiple partitions, instead of the entire table or one partition at a time. You include comparison operators other than = in the PARTITION clause, and the COMPUTE INCREMENTAL STATS statement applies to all partitions that match the comparison expression.

For example, the INT_PARTITIONS table contains 4 partitions. The following COMPUTE INCREMENTAL STATS statements affect some but not all partitions, as indicated by the Updated n partition(s) messages. The partitions that are affected depend on values in the partition key column X that match the comparison expression in the PARTITION clause.

show partitions int_partitions;
+-------+-------+--------+------+--------------+-------------------+---------+...
| x     | #Rows | #Files | Size | Bytes Cached | Cache Replication | Format  |...
+-------+-------+--------+------+--------------+-------------------+---------+...
| 99    | -1    | 0      | 0B   | NOT CACHED   | NOT CACHED        | PARQUET |...
| 120   | -1    | 0      | 0B   | NOT CACHED   | NOT CACHED        | TEXT    |...
| 150   | -1    | 0      | 0B   | NOT CACHED   | NOT CACHED        | TEXT    |...
| 200   | -1    | 0      | 0B   | NOT CACHED   | NOT CACHED        | TEXT    |...
| Total | -1    | 0      | 0B   | 0B           |                   |         |...
+-------+-------+--------+------+--------------+-------------------+---------+...

compute incremental stats int_partitions partition (x < 100);
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 1 column(s). |
+-----------------------------------------+

compute incremental stats int_partitions partition (x in (100, 150, 200));
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 2 partition(s) and 1 column(s). |
+-----------------------------------------+

compute incremental stats int_partitions partition (x between 100 and 175);
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 2 partition(s) and 1 column(s). |
+-----------------------------------------+

compute incremental stats int_partitions partition (x in (100, 150, 200) or x < 100);
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 3 partition(s) and 1 column(s). |
+-----------------------------------------+

compute incremental stats int_partitions partition (x != 150);
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 3 partition(s) and 1 column(s). |
+-----------------------------------------+

Complex type considerations:

Currently, the statistics created by the COMPUTE STATS statement do not include information about complex type columns. The column stats metrics for complex columns are always shown as -1. For queries involving complex type columns, Impala uses heuristics to estimate the data distribution within such columns.

HBase considerations:

COMPUTE STATS works for HBase tables also. The statistics gathered for HBase tables are somewhat different than for HDFS-backed tables, but that metadata is still used for optimization when HBase tables are involved in join queries.

Amazon S3 considerations:

COMPUTE STATS also works for tables where data resides in the Amazon Simple Storage Service (S3). See Using Impala with the Amazon S3 Filesystem for details.

Performance considerations:

The statistics collected by COMPUTE STATS are used to optimize join queries INSERT operations into Parquet tables, and other resource-intensive kinds of SQL statements. See Table and Column Statistics for details.

For large tables, the COMPUTE STATS statement itself might take a long time and you might need to tune its performance. The COMPUTE STATS statement does not work with the EXPLAIN statement, or the SUMMARY command in impala-shell. You can use the PROFILE statement in impala-shell to examine timing information for the statement as a whole. If a basic COMPUTE STATS statement takes a long time for a partitioned table, consider switching to the COMPUTE INCREMENTAL STATS syntax so that only newly added partitions are analyzed each time.

Examples:

This example shows two tables, T1 and T2, with a small number distinct values linked by a parent-child relationship between T1.ID and T2.PARENT. T1 is tiny, while T2 has approximately 100K rows. Initially, the statistics includes physical measurements such as the number of files, the total size, and size measurements for fixed-length columns such as with the INT type. Unknown values are represented by -1. After running COMPUTE STATS for each table, much more information is available through the SHOW STATS statements. If you were running a join query involving both of these tables, you would need statistics for both tables to get the most effective optimization for the query.

[localhost:21000] > show table stats t1;
Query: show table stats t1
+-------+--------+------+--------+
| #Rows | #Files | Size | Format |
+-------+--------+------+--------+
| -1    | 1      | 33B  | TEXT   |
+-------+--------+------+--------+
Returned 1 row(s) in 0.02s
[localhost:21000] > show table stats t2;
Query: show table stats t2
+-------+--------+----------+--------+
| #Rows | #Files | Size     | Format |
+-------+--------+----------+--------+
| -1    | 28     | 960.00KB | TEXT   |
+-------+--------+----------+--------+
Returned 1 row(s) in 0.01s
[localhost:21000] > show column stats t1;
Query: show column stats t1
+--------+--------+------------------+--------+----------+----------+
| Column | Type   | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+--------+------------------+--------+----------+----------+
| id     | INT    | -1               | -1     | 4        | 4        |
| s      | STRING | -1               | -1     | -1       | -1       |
+--------+--------+------------------+--------+----------+----------+
Returned 2 row(s) in 1.71s
[localhost:21000] > show column stats t2;
Query: show column stats t2
+--------+--------+------------------+--------+----------+----------+
| Column | Type   | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+--------+------------------+--------+----------+----------+
| parent | INT    | -1               | -1     | 4        | 4        |
| s      | STRING | -1               | -1     | -1       | -1       |
+--------+--------+------------------+--------+----------+----------+
Returned 2 row(s) in 0.01s
[localhost:21000] > compute stats t1;
Query: compute stats t1
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 2 column(s). |
+-----------------------------------------+
Returned 1 row(s) in 5.30s
[localhost:21000] > show table stats t1;
Query: show table stats t1
+-------+--------+------+--------+
| #Rows | #Files | Size | Format |
+-------+--------+------+--------+
| 3     | 1      | 33B  | TEXT   |
+-------+--------+------+--------+
Returned 1 row(s) in 0.01s
[localhost:21000] > show column stats t1;
Query: show column stats t1
+--------+--------+------------------+--------+----------+----------+
| Column | Type   | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+--------+------------------+--------+----------+----------+
| id     | INT    | 3                | -1     | 4        | 4        |
| s      | STRING | 3                | -1     | -1       | -1       |
+--------+--------+------------------+--------+----------+----------+
Returned 2 row(s) in 0.02s
[localhost:21000] > compute stats t2;
Query: compute stats t2
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 2 column(s). |
+-----------------------------------------+
Returned 1 row(s) in 5.70s
[localhost:21000] > show table stats t2;
Query: show table stats t2
+-------+--------+----------+--------+
| #Rows | #Files | Size     | Format |
+-------+--------+----------+--------+
| 98304 | 1      | 960.00KB | TEXT   |
+-------+--------+----------+--------+
Returned 1 row(s) in 0.03s
[localhost:21000] > show column stats t2;
Query: show column stats t2
+--------+--------+------------------+--------+----------+----------+
| Column | Type   | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+--------+------------------+--------+----------+----------+
| parent | INT    | 3                | -1     | 4        | 4        |
| s      | STRING | 6                | -1     | 14       | 9.3      |
+--------+--------+------------------+--------+----------+----------+
Returned 2 row(s) in 0.01s

The following example shows how to use the INCREMENTAL clause, available in Impala 2.1.0 and higher. The COMPUTE INCREMENTAL STATS syntax lets you collect statistics for newly added or changed partitions, without rescanning the entire table.

-- Initially the table has no incremental stats, as indicated
-- 'false' under Incremental stats.
show table stats item_partitioned;
+-------------+-------+--------+----------+--------------+---------+------------------
| i_category  | #Rows | #Files | Size     | Bytes Cached | Format  | Incremental stats
+-------------+-------+--------+----------+--------------+---------+------------------
| Books       | -1    | 1      | 223.74KB | NOT CACHED   | PARQUET | false
| Children    | -1    | 1      | 230.05KB | NOT CACHED   | PARQUET | false
| Electronics | -1    | 1      | 232.67KB | NOT CACHED   | PARQUET | false
| Home        | -1    | 1      | 232.56KB | NOT CACHED   | PARQUET | false
| Jewelry     | -1    | 1      | 223.72KB | NOT CACHED   | PARQUET | false
| Men         | -1    | 1      | 231.25KB | NOT CACHED   | PARQUET | false
| Music       | -1    | 1      | 237.90KB | NOT CACHED   | PARQUET | false
| Shoes       | -1    | 1      | 234.90KB | NOT CACHED   | PARQUET | false
| Sports      | -1    | 1      | 227.97KB | NOT CACHED   | PARQUET | false
| Women       | -1    | 1      | 226.27KB | NOT CACHED   | PARQUET | false
| Total       | -1    | 10     | 2.25MB   | 0B           |         |
+-------------+-------+--------+----------+--------------+---------+------------------

-- After the first COMPUTE INCREMENTAL STATS,
-- all partitions have stats. The first
-- COMPUTE INCREMENTAL STATS scans the whole
-- table, discarding any previous stats from
-- a traditional COMPUTE STATS statement.
compute incremental stats item_partitioned;
+-------------------------------------------+
| summary                                   |
+-------------------------------------------+
| Updated 10 partition(s) and 21 column(s). |
+-------------------------------------------+
show table stats item_partitioned;
+-------------+-------+--------+----------+--------------+---------+------------------
| i_category  | #Rows | #Files | Size     | Bytes Cached | Format  | Incremental stats
+-------------+-------+--------+----------+--------------+---------+------------------
| Books       | 1733  | 1      | 223.74KB | NOT CACHED   | PARQUET | true
| Children    | 1786  | 1      | 230.05KB | NOT CACHED   | PARQUET | true
| Electronics | 1812  | 1      | 232.67KB | NOT CACHED   | PARQUET | true
| Home        | 1807  | 1      | 232.56KB | NOT CACHED   | PARQUET | true
| Jewelry     | 1740  | 1      | 223.72KB | NOT CACHED   | PARQUET | true
| Men         | 1811  | 1      | 231.25KB | NOT CACHED   | PARQUET | true
| Music       | 1860  | 1      | 237.90KB | NOT CACHED   | PARQUET | true
| Shoes       | 1835  | 1      | 234.90KB | NOT CACHED   | PARQUET | true
| Sports      | 1783  | 1      | 227.97KB | NOT CACHED   | PARQUET | true
| Women       | 1790  | 1      | 226.27KB | NOT CACHED   | PARQUET | true
| Total       | 17957 | 10     | 2.25MB   | 0B           |         |
+-------------+-------+--------+----------+--------------+---------+------------------

-- Add a new partition...
alter table item_partitioned add partition (i_category='Camping');
-- Add or replace files in HDFS outside of Impala,
-- rendering the stats for a partition obsolete.
!import_data_into_sports_partition.sh
refresh item_partitioned;
drop incremental stats item_partitioned partition (i_category='Sports');
-- Now some partitions have incremental stats
-- and some do not.
show table stats item_partitioned;
+-------------+-------+--------+----------+--------------+---------+------------------
| i_category  | #Rows | #Files | Size     | Bytes Cached | Format  | Incremental stats
+-------------+-------+--------+----------+--------------+---------+------------------
| Books       | 1733  | 1      | 223.74KB | NOT CACHED   | PARQUET | true
| Camping     | -1    | 1      | 408.02KB | NOT CACHED   | PARQUET | false
| Children    | 1786  | 1      | 230.05KB | NOT CACHED   | PARQUET | true
| Electronics | 1812  | 1      | 232.67KB | NOT CACHED   | PARQUET | true
| Home        | 1807  | 1      | 232.56KB | NOT CACHED   | PARQUET | true
| Jewelry     | 1740  | 1      | 223.72KB | NOT CACHED   | PARQUET | true
| Men         | 1811  | 1      | 231.25KB | NOT CACHED   | PARQUET | true
| Music       | 1860  | 1      | 237.90KB | NOT CACHED   | PARQUET | true
| Shoes       | 1835  | 1      | 234.90KB | NOT CACHED   | PARQUET | true
| Sports      | -1    | 1      | 227.97KB | NOT CACHED   | PARQUET | false
| Women       | 1790  | 1      | 226.27KB | NOT CACHED   | PARQUET | true
| Total       | 17957 | 11     | 2.65MB   | 0B           |         |
+-------------+-------+--------+----------+--------------+---------+------------------

-- After another COMPUTE INCREMENTAL STATS,
-- all partitions have incremental stats, and only the 2
-- partitions without incremental stats were scanned.
compute incremental stats item_partitioned;
+------------------------------------------+
| summary                                  |
+------------------------------------------+
| Updated 2 partition(s) and 21 column(s). |
+------------------------------------------+
show table stats item_partitioned;
+-------------+-------+--------+----------+--------------+---------+------------------
| i_category  | #Rows | #Files | Size     | Bytes Cached | Format  | Incremental stats
+-------------+-------+--------+----------+--------------+---------+------------------
| Books       | 1733  | 1      | 223.74KB | NOT CACHED   | PARQUET | true
| Camping     | 5328  | 1      | 408.02KB | NOT CACHED   | PARQUET | true
| Children    | 1786  | 1      | 230.05KB | NOT CACHED   | PARQUET | true
| Electronics | 1812  | 1      | 232.67KB | NOT CACHED   | PARQUET | true
| Home        | 1807  | 1      | 232.56KB | NOT CACHED   | PARQUET | true
| Jewelry     | 1740  | 1      | 223.72KB | NOT CACHED   | PARQUET | true
| Men         | 1811  | 1      | 231.25KB | NOT CACHED   | PARQUET | true
| Music       | 1860  | 1      | 237.90KB | NOT CACHED   | PARQUET | true
| Shoes       | 1835  | 1      | 234.90KB | NOT CACHED   | PARQUET | true
| Sports      | 1783  | 1      | 227.97KB | NOT CACHED   | PARQUET | true
| Women       | 1790  | 1      | 226.27KB | NOT CACHED   | PARQUET | true
| Total       | 17957 | 11     | 2.65MB   | 0B           |         |
+-------------+-------+--------+----------+--------------+---------+------------------

File format considerations:

The COMPUTE STATS statement works with tables created with any of the file formats supported by Impala. See How Impala Works with Hadoop File Formats for details about working with the different file formats. The following considerations apply to COMPUTE STATS depending on the file format of the table.

The COMPUTE STATS statement works with text tables with no restrictions. These tables can be created through either Impala or Hive.

The COMPUTE STATS statement works with Parquet tables. These tables can be created through either Impala or Hive.

The COMPUTE STATS statement works with Avro tables without restriction in CDH 5.4 / Impala 2.2 and higher. In earlier releases, COMPUTE STATS worked only for Avro tables created through Hive, and required the CREATE TABLE statement to use SQL-style column names and types rather than an Avro-style schema specification.

The COMPUTE STATS statement works with RCFile tables with no restrictions. These tables can be created through either Impala or Hive.

The COMPUTE STATS statement works with SequenceFile tables with no restrictions. These tables can be created through either Impala or Hive.

The COMPUTE STATS statement works with partitioned tables, whether all the partitions use the same file format, or some partitions are defined through ALTER TABLE to use different file formats.

Statement type: DDL

Cancellation: Certain multi-stage statements (CREATE TABLE AS SELECT and COMPUTE STATS) can be cancelled during some stages, when running INSERT or SELECT operations internally. To cancel this statement, use Ctrl-C from the impala-shell interpreter, the Cancel button from the Watch page in Hue, Actions > Cancel from the Queries list in Cloudera Manager, or Cancel from the list of in-flight queries (for a particular node) on the Queries tab in the Impala web UI (port 25000).

Restrictions:

Note: Prior to Impala 1.4.0, COMPUTE STATS counted the number of NULL values in each column and recorded that figure in the metastore database. Because Impala does not currently use the NULL count during query planning, Impala 1.4.0 and higher speeds up the COMPUTE STATS statement by skipping this NULL counting.

Internal details:

Behind the scenes, the COMPUTE STATS statement executes two statements: one to count the rows of each partition in the table (or the entire table if unpartitioned) through the COUNT(*) function, and another to count the approximate number of distinct values in each column through the NDV() 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 COMPUTE STATS statement.

HDFS permissions:

The user ID that the impalad daemon runs under, typically the impala 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 COMPUTE STATS; or all the files in partitions without incremental stats in the case of COMPUTE INCREMENTAL STATS. It must also have read and execute permissions for all relevant directories holding the data files. (Essentially, COMPUTE STATS requires the same permissions as the underlying SELECT queries it runs against the table.)

Kudu considerations:

The COMPUTE STATS 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 # Rows column of the output from SHOW TABLE STATS. That column always shows -1 for all Kudu tables.

Related information:

DROP STATS Statement, SHOW TABLE STATS Statement, SHOW COLUMN STATS Statement, Table and Column Statistics

Page generated May 18, 2018.