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.
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. 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.
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.
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
may 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 43.31. pg_statistic
Columns
Name | Type | References | Description |
---|---|---|---|
starelid |
oid |
|
The table or index that the described column belongs to |
staattnum |
int2 |
|
The number of the described column |
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 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 |
int2 |
A code number indicating the kind of statistics stored in the
N th “slot” of the
pg_statistic row
|
|
staop |
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 |
float4[] |
Numerical statistics of the appropriate kind for the
N th “slot”, or NULL if the slot
kind does not involve numerical values
|
|
stavalues |
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' type more specifically than
anyarray
|