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Table and Column Statistics

Impala can do better optimization for complex or multi-table queries when it has access to statistics about the volume of data and how the values are distributed. Impala uses this information to help parallelize and distribute the work for a query. The following sections describe the categories of statistics Impala can work with, and how to produce them and keep them up to date.

Originally, Impala relied on the Hive mechanism for collecting statistics, through the Hive ANALYZE TABLE statement which initiates a MapReduce job. For better user-friendliness and reliability, Impala implements its own COMPUTE STATS statement in Impala 1.2.2 and higher, along with the DROP STATS, SHOW TABLE STATS, and SHOW COLUMN STATS statements.

Continue reading:

Overview of Table Statistics

The Impala query planner can make use of statistics about entire tables and partitions. This information includes physical characteristics such as the number of rows, number of data files, the total size of the data files, and the file format. For partitioned tables, the numbers are calculated per partition, and as totals for the whole table. This metadata is stored in the metastore database, and can be updated by either Impala or Hive. If a number is not available, the value -1 is used as a placeholder. Some numbers, such as number and total sizes of data files, are always kept up to date because they can be calculated cheaply, as part of gathering HDFS block metadata.

The following example shows table stats for an unpartitioned Parquet table. The values for the number and sizes of files are always available. Initially, the number of rows is not known, because it requires a potentially expensive scan through the entire table, and so that value is displayed as -1. The COMPUTE STATS statement fills in any unknown table stats values.

show table stats parquet_snappy;
+-------+--------+---------+--------------+-------------------+---------+-------------------+...
| #Rows | #Files | Size    | Bytes Cached | Cache Replication | Format  | Incremental stats |...
+-------+--------+---------+--------------+-------------------+---------+-------------------+...
| -1    | 96     | 23.35GB | NOT CACHED   | NOT CACHED        | PARQUET | false             |...
+-------+--------+---------+--------------+-------------------+---------+-------------------+...

compute stats parquet_snappy;
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 6 column(s). |
+-----------------------------------------+


show table stats parquet_snappy;
+------------+--------+---------+--------------+-------------------+---------+-------------------+...
| #Rows      | #Files | Size    | Bytes Cached | Cache Replication | Format  | Incremental stats |...
+------------+--------+---------+--------------+-------------------+---------+-------------------+...
| 1000000000 | 96     | 23.35GB | NOT CACHED   | NOT CACHED        | PARQUET | false             |...
+------------+--------+---------+--------------+-------------------+---------+-------------------+...

Impala performs some optimizations using this metadata on its own, and other optimizations by using a combination of table and column statistics.

To check that table statistics are available for a table, and see the details of those statistics, use the statement SHOW TABLE STATS table_name. See SHOW Statement for details.

If you use the Hive-based methods of gathering statistics, see the Hive wiki for information about the required configuration on the Hive side. Cloudera recommends using the Impala COMPUTE STATS statement to avoid potential configuration and scalability issues with the statistics-gathering process.

If you run the Hive statement ANALYZE TABLE COMPUTE STATISTICS FOR COLUMNS, Impala can only use the resulting column statistics if the table is unpartitioned. Impala cannot use Hive-generated column statistics for a partitioned table.

Overview of Column Statistics

The Impala query planner can make use of statistics about individual columns when that metadata is available in the metastore database. This technique is most valuable for columns compared across tables in join queries, to help estimate how many rows the query will retrieve from each table. These statistics are also important for correlated subqueries using the EXISTS() or IN() operators, which are processed internally the same way as join queries.

The following example shows column stats for an unpartitioned Parquet table. The values for the maximum and average sizes of some types are always available, because those figures are constant for numeric and other fixed-size types. Initially, the number of distinct values is not known, because it requires a potentially expensive scan through the entire table, and so that value is displayed as -1. The same applies to maximum and average sizes of variable-sized types, such as STRING. The COMPUTE STATS statement fills in most unknown column stats values. (It does not record the number of NULL values, because currently Impala does not use that figure for query optimization.)

show column stats parquet_snappy;
+-------------+----------+------------------+--------+----------+----------+
| Column      | Type     | #Distinct Values | #Nulls | Max Size | Avg Size |
+-------------+----------+------------------+--------+----------+----------+
| id          | BIGINT   | -1               | -1     | 8        | 8        |
| val         | INT      | -1               | -1     | 4        | 4        |
| zerofill    | STRING   | -1               | -1     | -1       | -1       |
| name        | STRING   | -1               | -1     | -1       | -1       |
| assertion   | BOOLEAN  | -1               | -1     | 1        | 1        |
| location_id | SMALLINT | -1               | -1     | 2        | 2        |
+-------------+----------+------------------+--------+----------+----------+

compute stats parquet_snappy;
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 6 column(s). |
+-----------------------------------------+

show column stats parquet_snappy;
+-------------+----------+------------------+--------+----------+-------------------+
| Column      | Type     | #Distinct Values | #Nulls | Max Size | Avg Size          |
+-------------+----------+------------------+--------+----------+-------------------+
| id          | BIGINT   | 183861280        | -1     | 8        | 8                 |
| val         | INT      | 139017           | -1     | 4        | 4                 |
| zerofill    | STRING   | 101761           | -1     | 6        | 6                 |
| name        | STRING   | 145636240        | -1     | 22       | 13.00020027160645 |
| assertion   | BOOLEAN  | 2                | -1     | 1        | 1                 |
| location_id | SMALLINT | 339              | -1     | 2        | 2                 |
+-------------+----------+------------------+--------+----------+-------------------+
  Note:

For column statistics to be effective in Impala, you also need to have table statistics for the applicable tables, as described in Overview of Table Statistics. When you use the Impala COMPUTE STATS statement, both table and column statistics are automatically gathered at the same time, for all columns in the table.

Currently, the COMPUTE STATS statement under CDH 4 does not store any statistics for DECIMAL columns. When Impala runs under CDH 5, which has better support for DECIMAL in the metastore database, COMPUTE STATS does collect statistics for DECIMAL columns and Impala uses the statistics to optimize query performance.

  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 make use of the NULL count during query planning, Impala 1.4.0 and higher speeds up the COMPUTE STATS statement by skipping this NULL counting.

To check whether column statistics are available for a particular set of columns, use the SHOW COLUMN STATS table_name statement, or check the extended EXPLAIN output for a query against that table that refers to those columns. See SHOW Statement and EXPLAIN Statement for details.

If you run the Hive statement ANALYZE TABLE COMPUTE STATISTICS FOR COLUMNS, Impala can only use the resulting column statistics if the table is unpartitioned. Impala cannot use Hive-generated column statistics for a partitioned table.

How Table and Column Statistics Work for Partitioned Tables

When you use Impala for "big data", you are highly likely to use partitioning for your biggest tables, the ones representing data that can be logically divided based on dates, geographic regions, or similar criteria. The table and column statistics are especially useful for optimizing queries on such tables. For example, a query involving one year might involve substantially more or less data than a query involving a different year, or a range of several years. Each query might be optimized differently as a result.

The following examples show how table and column stats work with a partitioned table. The table for this example is partitioned by year, month, and day. For simplicity, the sample data consists of 5 partitions, all from the same year and month. Table stats are collected independently for each partition. (In fact, the SHOW PARTITIONS statement displays exactly the same information as SHOW TABLE STATS for a partitioned table.) Column stats apply to the entire table, not to individual partitions. Because the partition key column values are represented as HDFS directories, their characteristics are typically known in advance, even when the values for non-key columns are shown as -1.

show partitions year_month_day;
+-------+-------+-----+-------+--------+---------+--------------+-------------------+---------+...
| year  | month | day | #Rows | #Files | Size    | Bytes Cached | Cache Replication | Format  |...
+-------+-------+-----+-------+--------+---------+--------------+-------------------+---------+...
| 2013  | 12    | 1   | -1    | 1      | 2.51MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 2   | -1    | 1      | 2.53MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 3   | -1    | 1      | 2.52MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 4   | -1    | 1      | 2.51MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 5   | -1    | 1      | 2.52MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| Total |       |     | -1    | 5      | 12.58MB | 0B           |                   |         |...
+-------+-------+-----+-------+--------+---------+--------------+-------------------+---------+...

show table stats year_month_day;
+-------+-------+-----+-------+--------+---------+--------------+-------------------+---------+...
| year  | month | day | #Rows | #Files | Size    | Bytes Cached | Cache Replication | Format  |...
+-------+-------+-----+-------+--------+---------+--------------+-------------------+---------+...
| 2013  | 12    | 1   | -1    | 1      | 2.51MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 2   | -1    | 1      | 2.53MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 3   | -1    | 1      | 2.52MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 4   | -1    | 1      | 2.51MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 5   | -1    | 1      | 2.52MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| Total |       |     | -1    | 5      | 12.58MB | 0B           |                   |         |...
+-------+-------+-----+-------+--------+---------+--------------+-------------------+---------+...

show column stats year_month_day;
+-----------+---------+------------------+--------+----------+----------+
| Column    | Type    | #Distinct Values | #Nulls | Max Size | Avg Size |
+-----------+---------+------------------+--------+----------+----------+
| id        | INT     | -1               | -1     | 4        | 4        |
| val       | INT     | -1               | -1     | 4        | 4        |
| zfill     | STRING  | -1               | -1     | -1       | -1       |
| name      | STRING  | -1               | -1     | -1       | -1       |
| assertion | BOOLEAN | -1               | -1     | 1        | 1        |
| year      | INT     | 1                | 0      | 4        | 4        |
| month     | INT     | 1                | 0      | 4        | 4        |
| day       | INT     | 5                | 0      | 4        | 4        |
+-----------+---------+------------------+--------+----------+----------+

compute stats year_month_day;
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 5 partition(s) and 5 column(s). |
+-----------------------------------------+

show table stats year_month_day;
+-------+-------+-----+--------+--------+---------+--------------+-------------------+---------+...
| year  | month | day | #Rows  | #Files | Size    | Bytes Cached | Cache Replication | Format  |...
+-------+-------+-----+--------+--------+---------+--------------+-------------------+---------+...
| 2013  | 12    | 1   | 93606  | 1      | 2.51MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 2   | 94158  | 1      | 2.53MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 3   | 94122  | 1      | 2.52MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 4   | 93559  | 1      | 2.51MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| 2013  | 12    | 5   | 93845  | 1      | 2.52MB  | NOT CACHED   | NOT CACHED        | PARQUET |...
| Total |       |     | 469290 | 5      | 12.58MB | 0B           |                   |         |...
+-------+-------+-----+--------+--------+---------+--------------+-------------------+---------+...

show column stats year_month_day;
+-----------+---------+------------------+--------+----------+-------------------+
| Column    | Type    | #Distinct Values | #Nulls | Max Size | Avg Size          |
+-----------+---------+------------------+--------+----------+-------------------+
| id        | INT     | 511129           | -1     | 4        | 4                 |
| val       | INT     | 364853           | -1     | 4        | 4                 |
| zfill     | STRING  | 311430           | -1     | 6        | 6                 |
| name      | STRING  | 471975           | -1     | 22       | 13.00160026550293 |
| assertion | BOOLEAN | 2                | -1     | 1        | 1                 |
| year      | INT     | 1                | 0      | 4        | 4                 |
| month     | INT     | 1                | 0      | 4        | 4                 |
| day       | INT     | 5                | 0      | 4        | 4                 |
+-----------+---------+------------------+--------+----------+-------------------+
  Note: Partitioned tables can grow so large that scanning the entire table, as the COMPUTE STATS statement does, is impractical just to update the statistics for a new partition. The standard COMPUTE STATS statement might take hours, or even days. That situation is where you switch to using incremental statistics, a feature available in CDH 5.3 / Impala 2.1 and higher. See Overview of Incremental Statistics for details about this feature and the COMPUTE INCREMENTAL STATS syntax.

If you run the Hive statement ANALYZE TABLE COMPUTE STATISTICS FOR COLUMNS, Impala can only use the resulting column statistics if the table is unpartitioned. Impala cannot use Hive-generated column statistics for a partitioned table.

Overview of Incremental Statistics

In Impala 2.1.0 and higher, you can use the syntax COMPUTE INCREMENTAL STATS and DROP INCREMENTAL STATS. The INCREMENTAL clauses work with incremental statistics, a specialized feature for partitioned tables that are large or frequently updated with new partitions.

When you compute incremental statistics for a partitioned table, by default Impala only processes those partitions that do not yet have incremental statistics. By processing only newly added partitions, you can keep statistics up to date for large partitioned tables, without incurring the overhead of reprocessing the entire table each time.

You can also compute or drop statistics for a single partition by including a PARTITION clause in the COMPUTE INCREMENTAL STATS or DROP INCREMENTAL STATS statement.

The metadata for incremental statistics is handled differently from the original style of statistics:

  • If you have an existing partitioned table for which you have already computed statistics, issuing COMPUTE INCREMENTAL STATS without a partition clause causes Impala to rescan the entire table. Once the incremental statistics are computed, any future COMPUTE INCREMENTAL STATS statements only scan any new partitions and any partitions where you performed DROP INCREMENTAL STATS.

  • The SHOW TABLE STATS and SHOW PARTITIONS statements now include an additional column showing whether incremental statistics are available for each column. A partition could already be covered by the original type of statistics based on a prior COMPUTE STATS statement, as indicated by a value other than -1 under the #Rows column. Impala query planning uses either kind of statistics when available.

  • COMPUTE INCREMENTAL STATS takes more time than COMPUTE STATS for the same volume of data. Therefore it is most suitable for tables with large data volume where new partitions are added frequently, making it impractical to run a full COMPUTE STATS operation for each new partition. For unpartitioned tables, or partitioned tables that are loaded once and not updated with new partitions, use the original COMPUTE STATS syntax.

  • COMPUTE INCREMENTAL STATS uses some memory in the catalogd process, proportional to the number of partitions and number of columns in the applicable table. The memory overhead is approximately 400 bytes for each column in each partition. This memory is reserved in the catalogd daemon, the statestored daemon, and in each instance of the impalad daemon.

  • In cases where new files are added to an existing partition, issue a REFRESH statement for the table, followed by a DROP INCREMENTAL STATS and COMPUTE INCREMENTAL STATS sequence for the changed partition.

  • The DROP INCREMENTAL STATS statement operates only on a single partition at a time. To remove statistics (whether incremental or not) from all partitions of a table, issue a DROP STATS statement with no INCREMENTAL or PARTITION clauses.

The following considerations apply to incremental statistics when the structure of an existing table is changed (known as schema evolution):

  • If you use an ALTER TABLE statement to drop a column, the existing statistics remain valid and COMPUTE INCREMENTAL STATS does not rescan any partitions.

  • If you use an ALTER TABLE statement to add a column, Impala rescans all partitions and fills in the appropriate column-level values the next time you run COMPUTE INCREMENTAL STATS.

  • If you use an ALTER TABLE statement to change the data type of a column, Impala rescans all partitions and fills in the appropriate column-level values the next time you run COMPUTE INCREMENTAL STATS.

  • If you use an ALTER TABLE statement to change the file format of a table, the existing statistics remain valid and a subsequent COMPUTE INCREMENTAL STATS does not rescan any partitions.

See COMPUTE STATS Statement and DROP STATS Statement for syntax details.

Generating Table and Column Statistics (COMPUTE STATS Statement)

To gather table statistics after loading data into a table or partition, you typically use the COMPUTE STATS statement. This statement is available in Impala 1.2.2 and higher. It gathers both table statistics and column statistics for all columns in a single operation. For large partitioned tables, where you frequently need to update statistics and it is impractical to scan the entire table each time, use the syntax COMPUTE INCREMENTAL STATS, which is available in CDH 5.3 / Impala 2.1 and higher.

If you use Hive as part of your ETL workflow, you can also use Hive to generate table and column statistics. You might need to do extra configuration within Hive itself, the metastore, or even set up a separate database to hold Hive-generated statistics. You might need to run multiple statements to generate all the necessary statistics. Therefore, prefer the Impala COMPUTE STATS statement where that technique is practical. For details about collecting statistics through Hive, see the Hive wiki.

If you run the Hive statement ANALYZE TABLE COMPUTE STATISTICS FOR COLUMNS, Impala can only use the resulting column statistics if the table is unpartitioned. Impala cannot use Hive-generated column statistics for a partitioned table.

For your very largest tables, you might find that COMPUTE STATS or even COMPUTE INCREMENTAL STATS take so long to scan the data that it is impractical to use them regularly. In such a case, after adding a partition or inserting new data, you can update just the number of rows property through an ALTER TABLE statement. See Setting the NUMROWS Value Manually through ALTER TABLE for details. Because the column statistics might be left in a stale state, do not use this technique as a replacement for COMPUTE STATS. Only use this technique if all other means of collecting statistics are impractical, or as a low-overhead operation that you run in between periodic COMPUTE STATS or COMPUTE INCREMENTAL STATS operations.

Detecting Missing Statistics

You can check whether a specific table has statistics using the SHOW TABLE STATS statement (for any table) or the SHOW PARTITIONS statement (for a partitioned table). Both statements display the same information. If a table or a partition does not have any statistics, the #Rows field contains -1. Once you compute statistics for the table or partition, the #Rows field changes to an accurate value.

The following example shows a table that initially does not have any statistics. The SHOW TABLE STATS statement displays different values for #Rows before and after the COMPUTE STATS operation.

[localhost:21000] > create table no_stats (x int);
[localhost:21000] > show table stats no_stats;
+-------+--------+------+--------------+--------+-------------------+
| #Rows | #Files | Size | Bytes Cached | Format | Incremental stats |
+-------+--------+------+--------------+--------+-------------------+
| -1    | 0      | 0B   | NOT CACHED   | TEXT   | false             |
+-------+--------+------+--------------+--------+-------------------+
[localhost:21000] > compute stats no_stats;
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 1 column(s). |
+-----------------------------------------+
[localhost:21000] > show table stats no_stats;
+-------+--------+------+--------------+--------+-------------------+
| #Rows | #Files | Size | Bytes Cached | Format | Incremental stats |
+-------+--------+------+--------------+--------+-------------------+
| 0     | 0      | 0B   | NOT CACHED   | TEXT   | false             |
+-------+--------+------+--------------+--------+-------------------+

The following example shows a similar progression with a partitioned table. Initially, #Rows is -1. After a COMPUTE STATS operation, #Rows changes to an accurate value. Any newly added partition starts with no statistics, meaning that you must collect statistics after adding a new partition.

[localhost:21000] > create table no_stats_partitioned (x int) partitioned by (year smallint);
[localhost:21000] > show table stats no_stats_partitioned;
+-------+-------+--------+------+--------------+--------+-------------------+
| year  | #Rows | #Files | Size | Bytes Cached | Format | Incremental stats |
+-------+-------+--------+------+--------------+--------+-------------------+
| Total | -1    | 0      | 0B   | 0B           |        |                   |
+-------+-------+--------+------+--------------+--------+-------------------+
[localhost:21000] > show partitions no_stats_partitioned;
+-------+-------+--------+------+--------------+--------+-------------------+
| year  | #Rows | #Files | Size | Bytes Cached | Format | Incremental stats |
+-------+-------+--------+------+--------------+--------+-------------------+
| Total | -1    | 0      | 0B   | 0B           |        |                   |
+-------+-------+--------+------+--------------+--------+-------------------+
[localhost:21000] > alter table no_stats_partitioned add partition (year=2013);
[localhost:21000] > compute stats no_stats_partitioned;
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 1 column(s). |
+-----------------------------------------+
[localhost:21000] > alter table no_stats_partitioned add partition (year=2014);
[localhost:21000] > show partitions no_stats_partitioned;
+-------+-------+--------+------+--------------+--------+-------------------+
| year  | #Rows | #Files | Size | Bytes Cached | Format | Incremental stats |
+-------+-------+--------+------+--------------+--------+-------------------+
| 2013  | 0     | 0      | 0B   | NOT CACHED   | TEXT   | false             |
| 2014  | -1    | 0      | 0B   | NOT CACHED   | TEXT   | false             |
| Total | 0     | 0      | 0B   | 0B           |        |                   |
+-------+-------+--------+------+--------------+--------+-------------------+
  Note: Because the default COMPUTE STATS statement creates and updates statistics for all partitions in a table, if you expect to frequently add new partitions, use the COMPUTE INCREMENTAL STATS syntax instead, which lets you compute stats for a single specified partition, or only for those partitions that do not already have incremental stats.

If checking each individual table is impractical, due to a large number of tables or views that hide the underlying base tables, you can also check for missing statistics for a particular query. Use the EXPLAIN statement to preview query efficiency before actually running the query. Use the query profile output available through the PROFILE command in impala-shell or the web UI to verify query execution and timing after running the query. Both the EXPLAIN plan and the PROFILE output display a warning if any tables or partitions involved in the query do not have statistics.

[localhost:21000] > create table no_stats (x int);
[localhost:21000] > explain select count(*) from no_stats;
+------------------------------------------------------------------------------------+
| Explain String                                                                     |
+------------------------------------------------------------------------------------+
| Estimated Per-Host Requirements: Memory=10.00MB VCores=1                           |
| WARNING: The following tables are missing relevant table and/or column statistics. |
| incremental_stats.no_stats                                                         |
|                                                                                    |
| 03:AGGREGATE [FINALIZE]                                                            |
| |  output: count:merge(*)                                                          |
| |                                                                                  |
| 02:EXCHANGE [UNPARTITIONED]                                                        |
| |                                                                                  |
| 01:AGGREGATE                                                                       |
| |  output: count(*)                                                                |
| |                                                                                  |
| 00:SCAN HDFS [incremental_stats.no_stats]                                          |
|    partitions=1/1 files=0 size=0B                                                  |
+------------------------------------------------------------------------------------+

Because Impala uses the partition pruning technique when possible to only evaluate certain partitions, if you have a partitioned table with statistics for some partitions and not others, whether or not the EXPLAIN statement shows the warning depends on the actual partitions used by the query. For example, you might see warnings or not for different queries against the same table:

-- No warning because all the partitions for the year 2012 have stats.
EXPLAIN SELECT ... FROM t1 WHERE year = 2012;

-- Missing stats warning because one or more partitions in this range
-- do not have stats.
EXPLAIN SELECT ... FROM t1 WHERE year BETWEEN 2006 AND 2009;

To confirm if any partitions at all in the table are missing statistics, you might explain a query that scans the entire table, such as SELECT COUNT(*) FROM table_name.

Keeping Statistics Up to Date

When the contents of a table or partition change significantly, recompute the stats for the relevant table or partition. The degree of change that qualifies as "significant" varies, depending on the absolute and relative sizes of the tables. Typically, if you add more than 30% more data to a table, it is worthwhile to recompute stats, because the differences in number of rows and number of distinct values might cause Impala to choose a different join order when that table is used in join queries. This guideline is most important for the largest tables. For example, adding 30% new data to a table containing 1 TB has a greater effect on join order than adding 30% to a table containing only a few megabytes, and the larger table has a greater effect on query performance if Impala chooses a suboptimal join order as a result of outdated statistics.

If you reload a complete new set of data for a table, but the number of rows and number of distinct values for each column is relatively unchanged from before, you do not need to recompute stats for the table.

If the statistics for a table are out of date, and the table's large size makes it impractical to recompute new stats immediately, you can use the DROP STATS statement to remove the obsolete statistics, making it easier to identify tables that need a new COMPUTE STATS operation.

For a large partitioned table, consider using the incremental stats feature available in Impala 2.1.0 and higher, as explained in Overview of Incremental Statistics. If you add a new partition to a table, it is worthwhile to recompute incremental stats, because the operation only scans the data for that one new partition.

Setting the NUMROWS Value Manually through ALTER TABLE

The most crucial piece of data in all the statistics is the number of rows in the table (for an unpartitioned or partitioned table) and for each partition (for a partitioned table). The COMPUTE STATS statement always gathers statistics about all columns, as well as overall table statistics. If it is not practical to do a full COMPUTE STATS or COMPUTE INCREMENTAL STATS operation after adding a partition or inserting data, or if you can see that Impala would produce a more efficient plan if the number of rows was different, you can manually set the number of rows through an ALTER TABLE statement:

-- Set total number of rows. Applies to both unpartitioned and partitioned tables.
alter table table_name set tblproperties('numRows'='new_value', 'STATS_GENERATED_VIA_STATS_TASK'='true');

-- Set total number of rows for a specific partition. Applies to partitioned tables only.
-- You must specify all the partition key columns in the PARTITION clause.
alter table table_name partition (keycol1=val1,keycol2=val2...) set tblproperties('numRows'='new_value', 'STATS_GENERATED_VIA_STATS_TASK'='true');

This statement avoids re-scanning any data files. (The requirement to include the STATS_GENERATED_VIA_STATS_TASK property is relatively new, as a result of the issue HIVE-8648 for the Hive metastore.)

create table analysis_data stored as parquet as select * from raw_data;
Inserted 1000000000 rows in 181.98s
compute stats analysis_data;
insert into analysis_data select * from smaller_table_we_forgot_before;
Inserted 1000000 rows in 15.32s
-- Now there are 1001000000 rows. We can update this single data point in the stats.
alter table analysis_data set tblproperties('numRows'='1001000000', 'STATS_GENERATED_VIA_STATS_TASK'='true');

For a partitioned table, update both the per-partition number of rows and the number of rows for the whole table:

-- If the table originally contained 1 million rows, and we add another partition with 30 thousand rows,
-- change the numRows property for the partition and the overall table.
alter table partitioned_data partition(year=2009, month=4) set tblproperties ('numRows'='30000', 'STATS_GENERATED_VIA_STATS_TASK'='true');
alter table partitioned_data set tblproperties ('numRows'='1030000', 'STATS_GENERATED_VIA_STATS_TASK'='true');

In practice, the COMPUTE STATS statement, or COMPUTE INCREMENTAL STATS for a partitioned table, should be fast and convenient enough that this technique is only useful for the very largest partitioned tables. Because the column statistics might be left in a stale state, do not use this technique as a replacement for COMPUTE STATS. Only use this technique if all other means of collecting statistics are impractical, or as a low-overhead operation that you run in between periodic COMPUTE STATS or COMPUTE INCREMENTAL STATS operations.

Examples of Using Table and Column Statistics with Impala

The following examples walk through a sequence of SHOW TABLE STATS, SHOW COLUMN STATS, ALTER TABLE, and SELECT and INSERT statements to illustrate various aspects of how Impala uses statistics to help optimize queries.

This example shows table and column statistics for the STORE column used in the TPC-DS benchmarks for decision support systems. It is a tiny table holding data for 12 stores. Initially, before any statistics are gathered by a COMPUTE STATS statement, most of the numeric fields show placeholder values of -1, indicating that the figures are unknown. The figures that are filled in are values that are easily countable or deducible at the physical level, such as the number of files, total data size of the files, and the maximum and average sizes for data types that have a constant size such as INT, FLOAT, and TIMESTAMP.

[localhost:21000] > show table stats store;
+-------+--------+--------+--------+
| #Rows | #Files | Size   | Format |
+-------+--------+--------+--------+
| -1    | 1      | 3.08KB | TEXT   |
+-------+--------+--------+--------+
Returned 1 row(s) in 0.03s
[localhost:21000] > show column stats store;
+--------------------+-----------+------------------+--------+----------+----------+
| Column             | Type      | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------------------+-----------+------------------+--------+----------+----------+
| s_store_sk         | INT       | -1               | -1     | 4        | 4        |
| s_store_id         | STRING    | -1               | -1     | -1       | -1       |
| s_rec_start_date   | TIMESTAMP | -1               | -1     | 16       | 16       |
| s_rec_end_date     | TIMESTAMP | -1               | -1     | 16       | 16       |
| s_closed_date_sk   | INT       | -1               | -1     | 4        | 4        |
| s_store_name       | STRING    | -1               | -1     | -1       | -1       |
| s_number_employees | INT       | -1               | -1     | 4        | 4        |
| s_floor_space      | INT       | -1               | -1     | 4        | 4        |
| s_hours            | STRING    | -1               | -1     | -1       | -1       |
| s_manager          | STRING    | -1               | -1     | -1       | -1       |
| s_market_id        | INT       | -1               | -1     | 4        | 4        |
| s_geography_class  | STRING    | -1               | -1     | -1       | -1       |
| s_market_desc      | STRING    | -1               | -1     | -1       | -1       |
| s_market_manager   | STRING    | -1               | -1     | -1       | -1       |
| s_division_id      | INT       | -1               | -1     | 4        | 4        |
| s_division_name    | STRING    | -1               | -1     | -1       | -1       |
| s_company_id       | INT       | -1               | -1     | 4        | 4        |
| s_company_name     | STRING    | -1               | -1     | -1       | -1       |
| s_street_number    | STRING    | -1               | -1     | -1       | -1       |
| s_street_name      | STRING    | -1               | -1     | -1       | -1       |
| s_street_type      | STRING    | -1               | -1     | -1       | -1       |
| s_suite_number     | STRING    | -1               | -1     | -1       | -1       |
| s_city             | STRING    | -1               | -1     | -1       | -1       |
| s_county           | STRING    | -1               | -1     | -1       | -1       |
| s_state            | STRING    | -1               | -1     | -1       | -1       |
| s_zip              | STRING    | -1               | -1     | -1       | -1       |
| s_country          | STRING    | -1               | -1     | -1       | -1       |
| s_gmt_offset       | FLOAT     | -1               | -1     | 4        | 4        |
| s_tax_percentage   | FLOAT     | -1               | -1     | 4        | 4        |
+--------------------+-----------+------------------+--------+----------+----------+
Returned 29 row(s) in 0.04s

With the Hive ANALYZE TABLE statement for column statistics, you had to specify each column for which to gather statistics. The Impala COMPUTE STATS statement automatically gathers statistics for all columns, because it reads through the entire table relatively quickly and can efficiently compute the values for all the columns. This example shows how after running the COMPUTE STATS statement, statistics are filled in for both the table and all its columns:

[localhost:21000] > compute stats store;
+------------------------------------------+
| summary                                  |
+------------------------------------------+
| Updated 1 partition(s) and 29 column(s). |
+------------------------------------------+
Returned 1 row(s) in 1.88s
[localhost:21000] > show table stats store;
+-------+--------+--------+--------+
| #Rows | #Files | Size   | Format |
+-------+--------+--------+--------+
| 12    | 1      | 3.08KB | TEXT   |
+-------+--------+--------+--------+
Returned 1 row(s) in 0.02s
[localhost:21000] > show column stats store;
+--------------------+-----------+------------------+--------+----------+-------------------+
| Column             | Type      | #Distinct Values | #Nulls | Max Size | Avg Size          |
+--------------------+-----------+------------------+--------+----------+-------------------+
| s_store_sk         | INT       | 12               | -1     | 4        | 4                 |
| s_store_id         | STRING    | 6                | -1     | 16       | 16                |
| s_rec_start_date   | TIMESTAMP | 4                | -1     | 16       | 16                |
| s_rec_end_date     | TIMESTAMP | 3                | -1     | 16       | 16                |
| s_closed_date_sk   | INT       | 3                | -1     | 4        | 4                 |
| s_store_name       | STRING    | 8                | -1     | 5        | 4.25              |
| s_number_employees | INT       | 9                | -1     | 4        | 4                 |
| s_floor_space      | INT       | 10               | -1     | 4        | 4                 |
| s_hours            | STRING    | 2                | -1     | 8        | 7.083300113677979 |
| s_manager          | STRING    | 7                | -1     | 15       | 12                |
| s_market_id        | INT       | 7                | -1     | 4        | 4                 |
| s_geography_class  | STRING    | 1                | -1     | 7        | 7                 |
| s_market_desc      | STRING    | 10               | -1     | 94       | 55.5              |
| s_market_manager   | STRING    | 7                | -1     | 16       | 14                |
| s_division_id      | INT       | 1                | -1     | 4        | 4                 |
| s_division_name    | STRING    | 1                | -1     | 7        | 7                 |
| s_company_id       | INT       | 1                | -1     | 4        | 4                 |
| s_company_name     | STRING    | 1                | -1     | 7        | 7                 |
| s_street_number    | STRING    | 9                | -1     | 3        | 2.833300113677979 |
| s_street_name      | STRING    | 12               | -1     | 11       | 6.583300113677979 |
| s_street_type      | STRING    | 8                | -1     | 9        | 4.833300113677979 |
| s_suite_number     | STRING    | 11               | -1     | 9        | 8.25              |
| s_city             | STRING    | 2                | -1     | 8        | 6.5               |
| s_county           | STRING    | 1                | -1     | 17       | 17                |
| s_state            | STRING    | 1                | -1     | 2        | 2                 |
| s_zip              | STRING    | 2                | -1     | 5        | 5                 |
| s_country          | STRING    | 1                | -1     | 13       | 13                |
| s_gmt_offset       | FLOAT     | 1                | -1     | 4        | 4                 |
| s_tax_percentage   | FLOAT     | 5                | -1     | 4        | 4                 |
+--------------------+-----------+------------------+--------+----------+-------------------+
Returned 29 row(s) in 0.04s

The following example shows how statistics are represented for a partitioned table. In this case, we have set up a table to hold the world's most trivial census data, a single STRING field, partitioned by a YEAR column. The table statistics include a separate entry for each partition, plus final totals for the numeric fields. The column statistics include some easily deducible facts for the partitioning column, such as the number of distinct values (the number of partition subdirectories).

localhost:21000] > describe census;
+------+----------+---------+
| name | type     | comment |
+------+----------+---------+
| name | string   |         |
| year | smallint |         |
+------+----------+---------+
Returned 2 row(s) in 0.02s
[localhost:21000] > show table stats census;
+-------+-------+--------+------+---------+
| year  | #Rows | #Files | Size | Format  |
+-------+-------+--------+------+---------+
| 2000  | -1    | 0      | 0B   | TEXT    |
| 2004  | -1    | 0      | 0B   | TEXT    |
| 2008  | -1    | 0      | 0B   | TEXT    |
| 2010  | -1    | 0      | 0B   | TEXT    |
| 2011  | 0     | 1      | 22B  | TEXT    |
| 2012  | -1    | 1      | 22B  | TEXT    |
| 2013  | -1    | 1      | 231B | PARQUET |
| Total | 0     | 3      | 275B |         |
+-------+-------+--------+------+---------+
Returned 8 row(s) in 0.02s
[localhost:21000] > show column stats census;
+--------+----------+------------------+--------+----------+----------+
| Column | Type     | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+----------+------------------+--------+----------+----------+
| name   | STRING   | -1               | -1     | -1       | -1       |
| year   | SMALLINT | 7                | -1     | 2        | 2        |
+--------+----------+------------------+--------+----------+----------+
Returned 2 row(s) in 0.02s

The following example shows how the statistics are filled in by a COMPUTE STATS statement in Impala.

[localhost:21000] > compute stats census;
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 3 partition(s) and 1 column(s). |
+-----------------------------------------+
Returned 1 row(s) in 2.16s
[localhost:21000] > show table stats census;
+-------+-------+--------+------+---------+
| year  | #Rows | #Files | Size | Format  |
+-------+-------+--------+------+---------+
| 2000  | -1    | 0      | 0B   | TEXT    |
| 2004  | -1    | 0      | 0B   | TEXT    |
| 2008  | -1    | 0      | 0B   | TEXT    |
| 2010  | -1    | 0      | 0B   | TEXT    |
| 2011  | 4     | 1      | 22B  | TEXT    |
| 2012  | 4     | 1      | 22B  | TEXT    |
| 2013  | 1     | 1      | 231B | PARQUET |
| Total | 9     | 3      | 275B |         |
+-------+-------+--------+------+---------+
Returned 8 row(s) in 0.02s
[localhost:21000] > show column stats census;
+--------+----------+------------------+--------+----------+----------+
| Column | Type     | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+----------+------------------+--------+----------+----------+
| name   | STRING   | 4                | -1     | 5        | 4.5      |
| year   | SMALLINT | 7                | -1     | 2        | 2        |
+--------+----------+------------------+--------+----------+----------+
Returned 2 row(s) in 0.02s

For examples showing how some queries work differently when statistics are available, see Examples of Join Order Optimization. You can see how Impala executes a query differently in each case by observing the EXPLAIN output before and after collecting statistics. Measure the before and after query times, and examine the throughput numbers in before and after SUMMARY or PROFILE output, to verify how much the improved plan speeds up performance.

Page generated August 6, 2016.