Oracle and PostgreSQL table statistics

With Amazon DMS, you can analyze table statistics for your Oracle and PostgreSQL databases to optimize query performance and storage utilization. Table statistics provide information about the data distribution and storage characteristics of database tables, including row counts, data sizes, and index usage.

Oracle usage

Table statistics are one of the important aspects affecting SQL query performance. They enable the query optimizer to make informed assumptions when deciding how to generate the execution plan for each query. Oracle provides the DBMS_STATS package to manage and control the table statistics, which can be collected automatically or manually.

The following statistics are usually collected on database tables and indexes:

Automatic optimizer statistics collection

By default, Oracle collects table and index statistics during predefined maintenance windows using the database scheduler and automated maintenance tasks. The automatic statistics collection mechanism uses Oracle’s data modification monitoring feature that tracks the approximate number of INSERT, UPDATE, and DELETE statements to determine which table statistics should be collected.

Oracle 19 now allows to gather real-time statistics on tables during regular UPDATE, INSERT, and DELETE operations, which ensures that statistics are always up-to-date and are not going stale.

Oracle 19 also introduces High-Frequency Automatic Optimizer Statistics Collection with an ability to set up automatic task that will collect statistics for stale objects.

Manual optimizer statistics collection

When the automatic statistics collection is not suitable for a particular use case, the optimizer statistics collection can be performed manually at several levels.

Examples

Collect statistics at the table level from the HR schema and the EMPLOYEES table.

BEGIN
DBMS_STATS.GATHER_TABLE_STATS('HR','EMPLOYEES');
END;
/

PL/SQL procedure successfully completed.

Collect statistics at a specific column level from the HR schema, the EMPLOYEES table, and the DEPARTMENT_ID column.

BEGIN
DBMS_STATS.GATHER_TABLE_STATS('HR','EMPLOYEES',
METHOD_OPT=>'FOR COLUMNS department_id');
END;
/

PL/SQL procedure successfully completed.

For more information, see Optimizer Statistics Concepts in the Oracle documentation.

PostgreSQL usage

Use the ANALYZE command to collect statistics about a database, a table or a specific table column. The PostgreSQL ANALYZE command collects table statistics which support generation of efficient query execution plans by the query planner.

PostgreSQL automatic statistics collection

By default, PostgreSQL is configured with an autovacuum daemon, which automates the execution of statistics collection using the ANALYZE commands (in addition to automation of the VACUUM command). The autovacuum daemon scans for tables which show signs of large modifications in data to collect the current statistics. Autovacuum is controlled by several parameters.

Individual tables have several storage parameters which can trigger autovacuum process sooner or later. These parameters, such as autovacuum_enabled, autovacuum_vacuum_threshold, and others can be set or changed using CREATE TABLE or ALTER TABLE statements.

ALTER TABLE custom_autovaccum
  SET (autovacuum_enabled = true,
    autovacuum_vacuum_cost_delay = 10ms,
    autovacuum_vacuum_scale_factor = 0.01,
    autovacuum_analyze_scale_factor = 0.005);

The preceding command enables autovaccum for the custom_autovaccum table and will specify the autovacuum process to sleep for 10 milliseconds each run.

It also specifies a 1% of the table size to be added to autovacuum_vacuum_threshold and 0.5% of the table size to be added to autovacuum_analyze_threshold when deciding whether to trigger a VACUUM.

For more information, see Automatic Vacuuming in the PostgreSQL documentation.

PostgreSQL manual statistics collection

PostgreSQL allows collecting statistics on-demand using the ANALYZE command at a database level, table level or column level.

Examples

Gather statistics for the entire database.

ANALYZE;

Gather statistics for a specific table. The VERBOSE keyword displays progress.

ANALYZE VERBOSE EMPLOYEES;

Gather statistics for a specific column.

ANALYZE EMPLOYEES (HIRE_DATE);

Specify the default_statistics_target parameter for an individual table column and reset it back to default.

ALTER TABLE EMPLOYEES ALTER COLUMN SALARY SET STATISTICS 150;

ALTER TABLE EMPLOYEES ALTER COLUMN SALARY SET STATISTICS -1;

Larger values increase the time needed to complete an ANALYZE, but improve the quality of the collected planner’s statistics which can potentially lead to better execution plans.

View the current (session / global) default_statistics_target, modify it to 150 and analyze the EMPLOYEES table.

SHOW default_statistics_target ;
SET default_statistics_target to 150;
ANALYZE EMPLOYEES;

View the last time statistics were collected for a table.

SELECT relname, last_analyze FROM pg_stat_all_tables;

Summary

BEGIN dbms_stats.gather_table_stats(
  ownname =>'hr',
  tabname => 'employees' , …
);
END;

ANALYZE EMPLOYEES;

BEGIN dbms_stats.gather_tabke_stats(
  ownname=>'HR', …
  ESTIMATE_PERCENT=>100);
END;

SET default_statistics_target to 150;
ANALYZE EMPLOYEES;

BEGIN
EXECUTE DBMS_STATS.GATHER_SCHEMA_STATS(
  ownname => 'HR');
END

ANALYZE;

select owner, table_name, last_analyzed;

select relname, last_analyze
  from pg_stat_all_tables;

For more information, see ANALYZE and The Autovacuum Daemon in the PostgreSQL documentation.