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 ::= 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 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 Statisticsabout 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 impala_perf_stats.html#perf_stats_incremental
for full usage details.
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.
In Impala 3.0 and lower, approximately 400 bytes of metadata per column per partition are needed for caching. Tables with a big number of partitions and many columns can add up to a significant memory overhead as the metadata 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 exceeds 2 GB, you might experience service downtime. In Impala 3.1 and higher, the issue was alleviated with an improved handling of incremental stats.
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.
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:
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 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 Amazon S3 Object Store 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 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, 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:
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 only computes the number of rows for the whole Kudu table,
partition level row counts are not available.
Related information:
DROP STATS Statement, SHOW TABLE STATS Statement, SHOW COLUMN STATS Statement, Table and Column Statistics