Identify simple column references in extended statistics

Until now, when defining extended statistics, everything except a plain
column reference was treated as complex expression. So for example "a"
was a column reference, but "(a)" would be an expression. In most cases
this does not matter much, but there were a couple strange consequences.
For example

    CREATE STATISTICS s ON a FROM t;

would fail, because extended stats require at least two columns. But

    CREATE STATISTICS s ON (a) FROM t;

would succeed, because that requirement does not apply to expressions.
Moreover, that statistics object is useless - the optimizer will always
use the regular statistics collected for attribute "a".

So do a bit more work to identify those expressions referencing a single
column, and translate them to a simple column reference. Backpatch to
14, where support for extended statistics on expressions was introduced.

Reported-by: Justin Pryzby
Backpatch-through: 14
Discussion: https://postgr.es/m/20210816013255.GS10479%40telsasoft.com

2 files changed, 3 insertions, 0 deletions

diff --git a/src/test/regress/expected/stats_ext.out b/src/test/regress/expected/stats_ext.out
index a7f12e989dd..c60ba45aba8 100644
--- a/src/test/regress/expected/stats_ext.out
+++ b/src/test/regress/expected/stats_ext.out
@@ -55,6 +55,8 @@ ERROR:  duplicate expression in statistics definition
 CREATE STATISTICS tst (unrecognized) ON x, y FROM ext_stats_test;
 ERROR:  unrecognized statistics kind "unrecognized"
 -- incorrect expressions
+CREATE STATISTICS tst ON (y) FROM ext_stats_test; -- single column reference
+ERROR:  extended statistics require at least 2 columns
 CREATE STATISTICS tst ON y + z FROM ext_stats_test; -- missing parentheses
 ERROR:  syntax error at or near "+"
 LINE 1: CREATE STATISTICS tst ON y + z FROM ext_stats_test;
diff --git a/src/test/regress/sql/stats_ext.sql b/src/test/regress/sql/stats_ext.sql
index 555b9473bbe..6fb37962a72 100644
--- a/src/test/regress/sql/stats_ext.sql
+++ b/src/test/regress/sql/stats_ext.sql
@@ -41,6 +41,7 @@ CREATE STATISTICS tst ON (x || 'x'), (x || 'x'), (y + 1), (x || 'x'), (x || 'x')
 CREATE STATISTICS tst ON (x || 'x'), (x || 'x'), y FROM ext_stats_test;
 CREATE STATISTICS tst (unrecognized) ON x, y FROM ext_stats_test;
 -- incorrect expressions
+CREATE STATISTICS tst ON (y) FROM ext_stats_test; -- single column reference
 CREATE STATISTICS tst ON y + z FROM ext_stats_test; -- missing parentheses
 CREATE STATISTICS tst ON (x, y) FROM ext_stats_test; -- tuple expression
 DROP TABLE ext_stats_test;