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 uptodate.
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 nonexpression 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 

  The table or index that the described column belongs to 

  The number of the described column 

  If true, the stats include inheritance child columns, not just the values in the specified relation 

  The fraction of the column's entries that are null 

  The average stored width, in bytes, of nonnull entries 

  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 

  A code number indicating the kind of statistics stored in the _ 

  An operator used to derive the statistics stored in the _ 

  Numerical statistics of the appropriate kind for the _ 

  Column data values of the appropriate kind for the _ 