ANALYZE

Nombre

ANALYZE -- colecta estadísticas sobre una base de datos

Sinopsis

ANALYZE [ VERBOSE ] [ tabla [ ( columna [,  ...] ) ] ]

Descripción

ANALYZE colecta estadísticas sobre los contenidos de las tablas en la base de datos, y almacena los resultados en el catálogo del sistema  pg_statistic. Posteriormente, el planeador de consultas usa estas estadísticas para ayudar a determinar el plan de ejecución más eficiente para las consultas.

Sin parámetros, ANALYZE examina cada tabla en la base de datos actual. Con un parámetro, ANALYZE examina solo una tabla. Además, se puede dar una lista de columna en cuyo caso solo las estadísticas para dichas columnas son colectadas.

Parámetros

VERBOSE
Habilita la visualización de mensajes de progreso.
tabla
El nombre (posiblemente calificado por esquema) de una tabla específica para anlizar. Por defecto se toman todas las tablas de la base de datos actual.
columna
El nombre de una columna específica a analizar. Por defecto se toman todas las columnas.

Salida

Cuando se especifica VERBOSE, ANALYZE emite mensajes de progreso para indicar que tabla está siendo actualmente procesada. También se imprimen varias estadísticas sobre las tablas.

Notes

In the default PostgreSQL configuration,  The Autovacuum Daemon takes care of automatic analyzing of tables when they are first loaded with data, and as they change throughout regular operation. When autovacuum is disabled, it is a good idea to run ANALYZE periodically, or just after making major changes in the contents of a table. Accurate statistics will help the planner to choose the most appropriate query plan, and thereby improve the speed of query processing. A common strategy is to run  VACUUM and ANALYZE once a day during a low-usage time of day.

ANALYZE requires only a read lock on the target table, so it can run in parallel with other activity on the table.

The statistics collected by ANALYZE usually include a list of some of the most common values in each column and a histogram showing the approximate data distribution in each column. One or both of these can be omitted if ANALYZE deems them uninteresting (for example, in a unique-key column, there are no common values) or if the column data type does not support the appropriate operators. There is more information about the statistics in  Chapter 23.

For large tables, ANALYZE takes a random sample of the table contents, rather than examining every row. This allows even very large tables to be analyzed in a small amount of time. Note, however, that the statistics are only approximate, and will change slightly each time ANALYZE is run, even if the actual table contents did not change. This might result in small changes in the planner's estimated costs shown by  EXPLAIN. In rare situations, this non-determinism will cause the planner's choices of query plans to change after ANALYZE is run. To avoid this, raise the amount of statistics collected by ANALYZE, as described below.

The extent of analysis can be controlled by adjusting the  default_statistics_target configuration variable, or on a column-by-column basis by setting the per-column statistics target with ALTER TABLE ... ALTER COLUMN ... SET STATISTICS (see  ALTER TABLE). The target value sets the maximum number of entries in the most-common-value list and the maximum number of bins in the histogram. The default target value is 100, but this can be adjusted up or down to trade off accuracy of planner estimates against the time taken for ANALYZE and the amount of space occupied in pg_statistic. In particular, setting the statistics target to zero disables collection of statistics for that column. It might be useful to do that for columns that are never used as part of the WHERE, GROUP BY, or ORDER BY clauses of queries, since the planner will have no use for statistics on such columns.

The largest statistics target among the columns being analyzed determines the number of table rows sampled to prepare the statistics. Increasing the target causes a proportional increase in the time and space needed to do ANALYZE.

Compatibility

There is no ANALYZE statement in the SQL standard.

See Also

 VACUUM,  vacuumdb,  Cost-Based Vacuum Delay,  The Autovacuum Daemon