pg_statistic

The pg_statistic system catalog table stores statistical data about the contents of the database. Entries are created by ANALYZE and subsequently used by the query optimizer. There is one entry for each table column that has been analyzed. Note that all the statistical data is inherently approximate, even assuming that it is up to date.

pg_statistic also stores statistical data about the values of index expressions. These are described as if they were actual data columns; in particular, starelid references the index. However, no entry is made for an ordinary non-expression index column, since it would be redundant with the entry for the underlying table column. Currently, entries for index expressions always have stainherit = false.

When stainherit = false, there is normally one entry for each table column that has been analyzed. If the table has inheritance children, Greengage DB creates a second entry with stainherit = true. This row represents the column’s statistics over the inheritance tree, for example, statistics for the data you would see with SELECT column FROM table*, whereas the stainherit = false row represents the results of SELECT column FROM ONLY table.

Since different kinds of statistics may be appropriate for different kinds of data, pg_statistic is designed not to assume very much about what sort of statistics it stores. Only extremely general statistics (such as nullness) are given dedicated columns in pg_statistic. Everything else is stored in slots, which are groups of associated columns whose content is identified by a code number in one of the slot’s columns.

Statistical information about a table’s contents should be considered sensitive (for example, minimum and maximum values of a salary column). pg_stats is a publicly readable view on pg_statistic that only exposes information about those tables that are readable by the current user.

CAUTION

Diagnostic tools such as gpsd and minirepro collect sensitive information from pg_statistic, such as histogram boundaries, in a clear, readable form. Always review the output files of these utilities to ensure that the contents are acceptable for transport outside of the database in your organization.

Column Type References Description

Column	Type	References	Description
starelid	oid	pg_class.oid	The table or index that the described column belongs to
staattnum	int2	pg_attribute.attnum	The number of the described column
stainherit	boolean	—	If `true`, the statistics include inheritance child columns, not just the values in the specified relations
stanullfrac	float4	—	The fraction of the column’s entries that are `NULL`
stawidth	integer	—	The average stored width, in bytes, of non-`NULL` entries
stadistinct	float4	—	The number of distinct non-`NULL` data values in the column. A value greater than zero is the actual number of distinct values. A value less than zero is the negative of a fraction of the number of rows in the table (for example, a column in which values appear about twice on the average could be represented by `stadistinct = -0.5`). A zero value means the number of distinct values is unknown
stakindN	int2	—	A code number indicating the kind of statistics stored in the N-th slot of the `pg_statistic` row
staopN	oid	pg_operator.oid	An operator used to derive the statistics stored in the N-th slot. For example, a histogram slot would show the `<` operator that defines the sort order of the data
stanumbersN	float4[]	—	Numerical statistics of the appropriate kind for the N-th slot, or `NULL` if the slot kind does not involve numerical values
stavaluesN	anyarray	—	Column data values of the appropriate kind for the N-th slot, or `NULL` if the slot kind does not store any data values. Each array’s element values are actually of the specific column’s data type, so there is no way to define these columns' types more specifically than `anyarray`

starelid

oid

pg_class.oid

The table or index that the described column belongs to

staattnum

int2

pg_attribute.attnum

The number of the described column

stainherit

boolean

—

If true, the statistics include inheritance child columns, not just the values in the specified relations

stanullfrac

float4

—

The fraction of the column’s entries that are NULL

stawidth

integer

—

The average stored width, in bytes, of non-NULL entries

stadistinct

float4

—

The number of distinct non-NULL data values in the column. A value greater than zero is the actual number of distinct values. A value less than zero is the negative of a fraction of the number of rows in the table (for example, a column in which values appear about twice on the average could be represented by stadistinct = -0.5). A zero value means the number of distinct values is unknown

stakind*N*

int2

—

A code number indicating the kind of statistics stored in the N-th slot of the pg_statistic row

staop*N*

oid

pg_operator.oid

An operator used to derive the statistics stored in the N-th slot. For example, a histogram slot would show the < operator that defines the sort order of the data

stanumbers*N*

float4[]

—

Numerical statistics of the appropriate kind for the N-th slot, or NULL if the slot kind does not involve numerical values

stavalues*N*

anyarray

—

Column data values of the appropriate kind for the N-th slot, or NULL if the slot kind does not store any data values. Each array’s element values are actually of the specific column’s data type, so there is no way to define these columns' types more specifically than anyarray

pg_stat_last_shoperation

pg_tablespace