pg_statistic
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
, whereas the
column
FROM table
*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 

starelid 
oid 

The table or index that the described column belongs to 
staattnum 
int2 

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. 

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,
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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