On Sat, Jan 16, 2021 at 05:48:43PM +0100, Tomas Vondra wrote: > + <entry role="catalog_table_entry"><para role="column_definition"> > + <structfield>expr</structfield> <type>text</type> > + </para> > + <para> > + Expression the extended statistics is defined on > + </para></entry>
Expression the extended statistics ARE defined on Or maybe say "on which the extended statistics are defined" > + <para> > + The <command>CREATE STATISTICS</command> command has two basic forms. The > + simple variant allows to build statistics for a single expression, does .. ALLOWS BUILDING statistics for a single expression, AND does (or BUT does) > + Expression statistics are per-expression and are similar to creating an > + index on the expression, except that they avoid the overhead of the index. Maybe say "overhead of index maintenance" > + All functions and operators used in a statistics definition must be > + <quote>immutable</quote>, that is, their results must depend only on > + their arguments and never on any outside influence (such as > + the contents of another table or the current time). This restriction say "outside factor" or "external factor" > + results of those expression, and uses default estimates as illustrated > + by the first query. The planner also does not realize the value of the realize THAT > + second column fully defines the value of the other column, because date > + truncated to day still identifies the month. Then expression and > + ndistinct statistics are built on those two columns: I got an error doing this: CREATE TABLE t AS SELECT generate_series(1,9) AS i; CREATE STATISTICS s ON (i+1) ,(i+1+0) FROM t; ANALYZE t; SELECT i+1 FROM t GROUP BY 1; ERROR: corrupt MVNDistinct entry -- Justin