The catalog pg_statistic stores statistical data about the contents of the database. Entries are created by ANALYZE and subsequently used by the query planner. Note that all the statistical data is inherently approximate, even assuming that it is up-to-date.
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 50-46. pg_statistic Columns
|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 Nth "slot" of the pg_statistic row.|
|staopN||oid||pg_operator.oid||An operator used to derive the statistics stored in the Nth "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 Nth "slot", or null if the slot kind does not involve numerical values|
|stavaluesN||anyarray||Column data values of the appropriate kind for the Nth "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.|
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