Extended statistics on expressions

Allow defining extended statistics on expressions, not just just on
simple column references.  With this commit, expressions are supported
by all existing extended statistics kinds, improving the same types of
estimates. A simple example may look like this:

  CREATE TABLE t (a int);
  CREATE STATISTICS s ON mod(a,10), mod(a,20) FROM t;
  ANALYZE t;

The collected statistics are useful e.g. to estimate queries with those
expressions in WHERE or GROUP BY clauses:

  SELECT * FROM t WHERE mod(a,10) = 0 AND mod(a,20) = 0;

  SELECT 1 FROM t GROUP BY mod(a,10), mod(a,20);

This introduces new internal statistics kind 'e' (expressions) which is
built automatically when the statistics object definition includes any
expressions. This represents single-expression statistics, as if there
was an expression index (but without the index maintenance overhead).
The statistics is stored in pg_statistics_ext_data as an array of
composite types, which is possible thanks to 79f6a942bd.

CREATE STATISTICS allows building statistics on a single expression, in
which case in which case it's not possible to specify statistics kinds.

A new system view pg_stats_ext_exprs can be used to display expression
statistics, similarly to pg_stats and pg_stats_ext views.

ALTER TABLE ... ALTER COLUMN ... TYPE now treats indexes the same way it
treats indexes, i.e. it drops and recreates the statistics. This means
all statistics are reset, and we no longer try to preserve at least the
functional dependencies. This should not be a major issue in practice,
as the functional dependencies actually rely on per-column statistics,
which were always reset anyway.

Author: Tomas Vondra
Reviewed-by: Justin Pryzby, Dean Rasheed, Zhihong Yu
Discussion: https://postgr.es/m/ad7891d2-e90c-b446-9fe2-7419143847d7%40enterprisedb.com

1 files changed, 6 insertions, 0 deletions

diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index f869e159d63..03373d551fc 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -500,6 +500,7 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 		case EXPR_KIND_FUNCTION_DEFAULT:
 		case EXPR_KIND_INDEX_EXPRESSION:
 		case EXPR_KIND_INDEX_PREDICATE:
+		case EXPR_KIND_STATS_EXPRESSION:
 		case EXPR_KIND_ALTER_COL_TRANSFORM:
 		case EXPR_KIND_EXECUTE_PARAMETER:
 		case EXPR_KIND_TRIGGER_WHEN:
@@ -1741,6 +1742,9 @@ transformSubLink(ParseState *pstate, SubLink *sublink)
 		case EXPR_KIND_INDEX_PREDICATE:
 			err = _("cannot use subquery in index predicate");
 			break;
+		case EXPR_KIND_STATS_EXPRESSION:
+			err = _("cannot use subquery in statistics expression");
+			break;
 		case EXPR_KIND_ALTER_COL_TRANSFORM:
 			err = _("cannot use subquery in transform expression");
 			break;
@@ -3030,6 +3034,8 @@ ParseExprKindName(ParseExprKind exprKind)
 			return "index expression";
 		case EXPR_KIND_INDEX_PREDICATE:
 			return "index predicate";
+		case EXPR_KIND_STATS_EXPRESSION:
+			return "statistics expression";
 		case EXPR_KIND_ALTER_COL_TRANSFORM:
 			return "USING";
 		case EXPR_KIND_EXECUTE_PARAMETER: