diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/ref/alter_table.sgml | 34 |
1 files changed, 16 insertions, 18 deletions
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml index 8867da6c693..bea9f90138b 100644 --- a/doc/src/sgml/ref/alter_table.sgml +++ b/doc/src/sgml/ref/alter_table.sgml @@ -376,24 +376,22 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM <literal>n_distinct_inherited</literal>, which override the number-of-distinct-values estimates made by subsequent <link linkend="sql-analyze"><command>ANALYZE</command></link> - operations. <literal>n_distinct</literal> affects the statistics for the table - itself, while <literal>n_distinct_inherited</literal> affects the statistics - gathered for the table plus its inheritance children. When set to a - positive value, <command>ANALYZE</command> will assume that the column contains - exactly the specified number of distinct nonnull values. When set to a - negative value, which must be greater - than or equal to -1, <command>ANALYZE</command> will assume that the number of - distinct nonnull values in the column is linear in the size of the - table; the exact count is to be computed by multiplying the estimated - table size by the absolute value of the given number. For example, - a value of -1 implies that all values in the column are distinct, while - a value of -0.5 implies that each value appears twice on the average. - This can be useful when the size of the table changes over time, since - the multiplication by the number of rows in the table is not performed - until query planning time. Specify a value of 0 to revert to estimating - the number of distinct values normally. For more information on the use - of statistics by the <productname>PostgreSQL</productname> query - planner, refer to <xref linkend="planner-stats"/>. + operations. <literal>n_distinct</literal> affects the statistics for the + table itself, while <literal>n_distinct_inherited</literal> affects the + statistics gathered for the table plus its inheritance children, and for + the statistics gathered for partitioned tables. When the value + specified is a positive value, the query planner will assume that the + column contains exactly the specified number of distinct nonnull values. + Fractional values may also be specified by using values below 0 and + above or equal to -1. This instructs the query planner to estimate the + number of distinct values by multiplying the absolute value of the + specified number by the estimated number of rows in the table. For + example, a value of -1 implies that all values in the column are + distinct, while a value of -0.5 implies that each value appears twice on + average. This can be useful when the size of the table changes over + time. For more information on the use of statistics by the + <productname>PostgreSQL</productname> query planner, refer to + <xref linkend="planner-stats"/>. </para> <para> Changing per-attribute options acquires a |