51.50. pg_statistic

系統目錄 pg_statistic 儲存有關資料庫內容的統計數據。內容項目是由 ANALYZE 所建立的，隨後由查詢計劃程序使用。請注意，所有統計數據本質上都是趨近的，即使假設上它是最新的數據。

Normally there is one entry, with stainherit = false, for each table column that has been analyzed. If the table has inheritance children, a second entry with stainherit = true is also created. This row represents the column's statistics over the inheritance tree, i.e., statistics for the data you'd see with SELECT column FROM table*, whereas the stainherit = false row represents the results of SELECT column FROM ONLY table.

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. No entry is made for an ordinary non-expression index column, however, since it would be redundant with the entry for the underlying table column. Currently, entries for index expressions always have stainherit = false.

Since different kinds of statistics might 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. For more information see src/include/catalog/pg_statistic.h.

pg_statistic should not be readable by the public, since even statistical information about a table's contents might be considered sensitive. (Example: minimum and maximum values of a salary column might be quite interesting.) pg_stats is a publicly readable view on pg_statistic that only exposes information about those tables that are readable by the current user.

Table 51.50. pg_statistic Columns

Name	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	bool		If true, the stats include inheritance child columns, not just the values in the specified relation
stanullfrac	float4		The fraction of the column's entries that are null
stawidth	int4		The average stored width, in bytes, of nonnull entries
stadistinct	float4		The number of distinct nonnull 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 multiplier for the number of rows in the table; for example, a column in which about 80% of the values are nonnull and each nonnull value appears about twice on average could be represented by stadistinct = -0.4. 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, or a related type such as an array's element type, so there is no way to define these columns' type more specifically than anyarray.