'\" t
.\"     Title: CREATE STATISTICS
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 2023
.\"    Manual: PostgreSQL 11.22 Documentation
.\"    Source: PostgreSQL 11.22
.\"  Language: English
.\"
.TH "CREATE STATISTICS" "7" "2023" "PostgreSQL 11.22" "PostgreSQL 11.22 Documentation"
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.SH "NAME"
CREATE_STATISTICS \- define extended statistics
.SH "SYNOPSIS"
.sp
.nf
CREATE STATISTICS [ IF NOT EXISTS ] \fIstatistics_name\fR
    [ ( \fIstatistics_kind\fR [, \&.\&.\&. ] ) ]
    ON \fIcolumn_name\fR, \fIcolumn_name\fR [, \&.\&.\&.]
    FROM \fItable_name\fR
.fi
.SH "DESCRIPTION"
.PP
\fBCREATE STATISTICS\fR
will create a new extended statistics object tracking data about the specified table, foreign table or materialized view\&. The statistics object will be created in the current database and will be owned by the user issuing the command\&.
.PP
If a schema name is given (for example,
CREATE STATISTICS myschema\&.mystat \&.\&.\&.) then the statistics object is created in the specified schema\&. Otherwise it is created in the current schema\&. The name of the statistics object must be distinct from the name of any other statistics object in the same schema\&.
.SH "PARAMETERS"
.PP
IF NOT EXISTS
.RS 4
Do not throw an error if a statistics object with the same name already exists\&. A notice is issued in this case\&. Note that only the name of the statistics object is considered here, not the details of its definition\&.
.RE
.PP
\fIstatistics_name\fR
.RS 4
The name (optionally schema\-qualified) of the statistics object to be created\&.
.RE
.PP
\fIstatistics_kind\fR
.RS 4
A statistics kind to be computed in this statistics object\&. Currently supported kinds are
ndistinct, which enables n\-distinct statistics, and
dependencies, which enables functional dependency statistics\&. If this clause is omitted, all supported statistics kinds are included in the statistics object\&. For more information, see
Section\ \&14.2.2
and
Section\ \&71.2\&.
.RE
.PP
\fIcolumn_name\fR
.RS 4
The name of a table column to be covered by the computed statistics\&. At least two column names must be given\&.
.RE
.PP
\fItable_name\fR
.RS 4
The name (optionally schema\-qualified) of the table containing the column(s) the statistics are computed on; see
\fBANALYZE\fR(7)
for an explanation of the handling of inheritance and partitions\&.
.RE
.SH "NOTES"
.PP
You must be the owner of a table to create a statistics object reading it\&. Once created, however, the ownership of the statistics object is independent of the underlying table(s)\&.
.PP
Extended statistics are not currently used by the planner for selectivity estimations made for table joins\&. This limitation will likely be removed in a future version of
PostgreSQL\&.
.SH "EXAMPLES"
.PP
Create table
t1
with two functionally dependent columns, i\&.e\&., knowledge of a value in the first column is sufficient for determining the value in the other column\&. Then functional dependency statistics are built on those columns:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE TABLE t1 (
    a   int,
    b   int
);

INSERT INTO t1 SELECT i/100, i/500
                 FROM generate_series(1,1000000) s(i);

ANALYZE t1;

\-\- the number of matching rows will be drastically underestimated:
EXPLAIN ANALYZE SELECT * FROM t1 WHERE (a = 1) AND (b = 0);

CREATE STATISTICS s1 (dependencies) ON a, b FROM t1;

ANALYZE t1;

\-\- now the row count estimate is more accurate:
EXPLAIN ANALYZE SELECT * FROM t1 WHERE (a = 1) AND (b = 0);
.fi
.if n \{\
.RE
.\}
.sp
Without functional\-dependency statistics, the planner would assume that the two
WHERE
conditions are independent, and would multiply their selectivities together to arrive at a much\-too\-small row count estimate\&. With such statistics, the planner recognizes that the
WHERE
conditions are redundant and does not underestimate the row count\&.
.SH "COMPATIBILITY"
.PP
There is no
\fBCREATE STATISTICS\fR
command in the SQL standard\&.
.SH "SEE ALSO"
ALTER STATISTICS (\fBALTER_STATISTICS\fR(7)), DROP STATISTICS (\fBDROP_STATISTICS\fR(7))