DROP STATS Statement
Removes the specified statistics from a table or partition. The statistics were originally created by the COMPUTE STATS or COMPUTE INCREMENTAL STATS statement.
Syntax:
DROP STATS [database_name.]table_name DROP INCREMENTAL STATS [database_name.]table_name PARTITION (partition_spec) partition_spec ::= partition_col=constant_value
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.
DROP STATS removes all statistics from the table, whether created by COMPUTE STATS or COMPUTE INCREMENTAL STATS.
DROP INCREMENTAL STATS only affects incremental statistics for a single partition, specified through the PARTITION clause. The incremental stats are marked as outdated, so that they are recomputed by the next COMPUTE INCREMENTAL STATS statement.
Usage notes:
You typically use this statement when the statistics for a table or a partition have become stale due to data files being added to or removed from the associated HDFS data directories, whether by manual HDFS operations or INSERT, INSERT OVERWRITE, or LOAD DATA statements, or adding or dropping partitions.
When a table or partition has no associated statistics, Impala treats it as essentially zero-sized when constructing the execution plan for a query. In particular, the statistics influence the order in which tables are joined in a join query. To ensure proper query planning and good query performance and scalability, make sure to run COMPUTE STATS or COMPUTE INCREMENTAL STATS on the table or partition after removing any stale statistics.
Dropping the statistics is not required for an unpartitioned table or a partitioned table covered by the original type of statistics. A subsequent COMPUTE STATS statement replaces any existing statistics with new ones, for all partitions, regardless of whether the old ones were outdated. Therefore, this statement was rarely used before the introduction of incremental statistics.
Dropping the statistics is required for a partitioned table containing incremental statistics, to make a subsequent COMPUTE INCREMENTAL STATS statement rescan an existing partition. See Table and Column Statistics for information about incremental statistics, a new feature available in Impala 2.1.0 and higher.
Statement type: DDL
Cancellation: Cannot be cancelled.
HDFS permissions:
The user ID that the impalad daemon runs under, typically the impala user, does not need any particular HDFS permissions to perform this statement. All read and write operations are on the metastore database, not HDFS files and directories.
Examples:
The following example shows a partitioned table that has associated statistics produced by the COMPUTE INCREMENTAL STATS statement, and how the situation evolves as statistics are dropped from specific partitions, then the entire table.
Initially, all table and column statistics are filled in.
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 | | +-------------+-------+--------+----------+--------------+---------+----------------- show column stats item_partitioned; +------------------+-----------+------------------+--------+----------+-------------- | Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size +------------------+-----------+------------------+--------+----------+-------------- | i_item_sk | INT | 19443 | -1 | 4 | 4 | i_item_id | STRING | 9025 | -1 | 16 | 16 | i_rec_start_date | TIMESTAMP | 4 | -1 | 16 | 16 | i_rec_end_date | TIMESTAMP | 3 | -1 | 16 | 16 | i_item_desc | STRING | 13330 | -1 | 200 | 100.302803039 | i_current_price | FLOAT | 2807 | -1 | 4 | 4 | i_wholesale_cost | FLOAT | 2105 | -1 | 4 | 4 | i_brand_id | INT | 965 | -1 | 4 | 4 | i_brand | STRING | 725 | -1 | 22 | 16.1776008605 | i_class_id | INT | 16 | -1 | 4 | 4 | i_class | STRING | 101 | -1 | 15 | 7.76749992370 | i_category_id | INT | 10 | -1 | 4 | 4 | i_manufact_id | INT | 1857 | -1 | 4 | 4 | i_manufact | STRING | 1028 | -1 | 15 | 11.3295001983 | i_size | STRING | 8 | -1 | 11 | 4.33459997177 | i_formulation | STRING | 12884 | -1 | 20 | 19.9799995422 | i_color | STRING | 92 | -1 | 10 | 5.38089990615 | i_units | STRING | 22 | -1 | 7 | 4.18690013885 | i_container | STRING | 2 | -1 | 7 | 6.99259996414 | i_manager_id | INT | 105 | -1 | 4 | 4 | i_product_name | STRING | 19094 | -1 | 25 | 18.0233001708 | i_category | STRING | 10 | 0 | -1 | -1 +------------------+-----------+------------------+--------+----------+--------------
To remove statistics for particular partitions, use the DROP INCREMENTAL STATS statement. After removing statistics for two partitions, the table-level statistics reflect that change in the #Rows and Incremental stats fields. The counts, maximums, and averages of the column-level statistics are unaffected.
drop incremental stats item_partitioned partition (i_category='Sports'); drop incremental stats item_partitioned partition (i_category='Electronics'); 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 | -1 | 1 | 232.67KB | NOT CACHED | PARQUET | false | 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 | 10 | 2.25MB | 0B | | +-------------+-------+--------+----------+--------------+---------+----------------- show column stats item_partitioned +------------------+-----------+------------------+--------+----------+-------------- | Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size +------------------+-----------+------------------+--------+----------+-------------- | i_item_sk | INT | 19443 | -1 | 4 | 4 | i_item_id | STRING | 9025 | -1 | 16 | 16 | i_rec_start_date | TIMESTAMP | 4 | -1 | 16 | 16 | i_rec_end_date | TIMESTAMP | 3 | -1 | 16 | 16 | i_item_desc | STRING | 13330 | -1 | 200 | 100.302803039 | i_current_price | FLOAT | 2807 | -1 | 4 | 4 | i_wholesale_cost | FLOAT | 2105 | -1 | 4 | 4 | i_brand_id | INT | 965 | -1 | 4 | 4 | i_brand | STRING | 725 | -1 | 22 | 16.1776008605 | i_class_id | INT | 16 | -1 | 4 | 4 | i_class | STRING | 101 | -1 | 15 | 7.76749992370 | i_category_id | INT | 10 | -1 | 4 | 4 | i_manufact_id | INT | 1857 | -1 | 4 | 4 | i_manufact | STRING | 1028 | -1 | 15 | 11.3295001983 | i_size | STRING | 8 | -1 | 11 | 4.33459997177 | i_formulation | STRING | 12884 | -1 | 20 | 19.9799995422 | i_color | STRING | 92 | -1 | 10 | 5.38089990615 | i_units | STRING | 22 | -1 | 7 | 4.18690013885 | i_container | STRING | 2 | -1 | 7 | 6.99259996414 | i_manager_id | INT | 105 | -1 | 4 | 4 | i_product_name | STRING | 19094 | -1 | 25 | 18.0233001708 | i_category | STRING | 10 | 0 | -1 | -1 +------------------+-----------+------------------+--------+----------+--------------
To remove all statistics from the table, whether produced by COMPUTE STATS or COMPUTE INCREMENTAL STATS, use the DROP STATS statement without the INCREMENTAL clause). Now, both table-level and column-level statistics are reset.
drop stats item_partitioned; 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 | | +-------------+-------+--------+----------+--------------+---------+------------------ show column stats item_partitioned +------------------+-----------+------------------+--------+----------+----------+ | Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size | +------------------+-----------+------------------+--------+----------+----------+ | i_item_sk | INT | -1 | -1 | 4 | 4 | | i_item_id | STRING | -1 | -1 | -1 | -1 | | i_rec_start_date | TIMESTAMP | -1 | -1 | 16 | 16 | | i_rec_end_date | TIMESTAMP | -1 | -1 | 16 | 16 | | i_item_desc | STRING | -1 | -1 | -1 | -1 | | i_current_price | FLOAT | -1 | -1 | 4 | 4 | | i_wholesale_cost | FLOAT | -1 | -1 | 4 | 4 | | i_brand_id | INT | -1 | -1 | 4 | 4 | | i_brand | STRING | -1 | -1 | -1 | -1 | | i_class_id | INT | -1 | -1 | 4 | 4 | | i_class | STRING | -1 | -1 | -1 | -1 | | i_category_id | INT | -1 | -1 | 4 | 4 | | i_manufact_id | INT | -1 | -1 | 4 | 4 | | i_manufact | STRING | -1 | -1 | -1 | -1 | | i_size | STRING | -1 | -1 | -1 | -1 | | i_formulation | STRING | -1 | -1 | -1 | -1 | | i_color | STRING | -1 | -1 | -1 | -1 | | i_units | STRING | -1 | -1 | -1 | -1 | | i_container | STRING | -1 | -1 | -1 | -1 | | i_manager_id | INT | -1 | -1 | 4 | 4 | | i_product_name | STRING | -1 | -1 | -1 | -1 | | i_category | STRING | 10 | 0 | -1 | -1 | +------------------+-----------+------------------+--------+----------+----------+
Related information:
COMPUTE STATS Statement, SHOW TABLE STATS Statement, SHOW COLUMN STATS Statement, Table and Column Statistics