Fix choose_best_statistics to check clauses individually

When picking the best extended statistics object for a list of clauses,
it's not enough to look at attnums extracted from the clause list as a
whole. Consider for example this query with OR clauses:

   SELECT * FROM t WHERE (t.a = 1) OR (t.b = 1) OR (t.c = 1)

with a statistics defined on columns (a,b). Relying on attnums extracted
from the whole OR clause, we'd consider the statistics usable. That does
not work, as we see the conditions as a single OR-clause, referencing an
attribute not covered by the statistic, leading to empty list of clauses
to be estimated using the statistics and an assert failure.

This changes choose_best_statistics to check which clauses are actually
covered, and only using attributes from the fully covered ones. For the
previous example this means the statistics object will not be considered
as compatible with the OR-clause.

Backpatch to 12, where MCVs were introduced. The issue does not affect
older versions because functional dependencies don't handle OR clauses.

Author: Tomas Vondra
Reviewed-by: Dean Rasheed
Reported-By: Manuel Rigger
Discussion: https://postgr.es/m/CA+u7OA7H5rcE2=8f263w4NZD6ipO_XOrYB816nuLXbmSTH9pQQ@mail.gmail.com
Backpatch-through: 12

src/test/regress/expected/stats_ext.out

@@ -517,6 +517,18 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <
          1 |     50
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+ estimated | actual 
+-----------+--------
+       343 |    200
+(1 row)
+
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+ estimated | actual 
+-----------+--------
+       343 |    200
+(1 row)
+
 -- create statistics
 CREATE STATISTICS mcv_lists_stats (mcv) ON a, b, c FROM mcv_lists;
 ANALYZE mcv_lists;
@@ -556,6 +568,19 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <
         50 |     50
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+ estimated | actual 
+-----------+--------
+       200 |    200
+(1 row)
+
+-- we can't use the statistic for OR clauses that are not fully covered (missing 'd' attribute)
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+ estimated | actual 
+-----------+--------
+       343 |    200
+(1 row)
+
 -- check change of unrelated column type does not reset the MCV statistics
 ALTER TABLE mcv_lists ALTER COLUMN d TYPE VARCHAR(64);
 SELECT d.stxdmcv IS NOT NULL

src/test/regress/sql/stats_ext.sql

@@ -332,6 +332,10 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a < 5 AND b <
 
 SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <= ''0'' AND c <= 4');
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+
 -- create statistics
 CREATE STATISTICS mcv_lists_stats (mcv) ON a, b, c FROM mcv_lists;
 
@@ -349,6 +353,11 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a < 5 AND b <
 
 SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <= ''0'' AND c <= 4');
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+
+-- we can't use the statistic for OR clauses that are not fully covered (missing 'd' attribute)
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+
 -- check change of unrelated column type does not reset the MCV statistics
 ALTER TABLE mcv_lists ALTER COLUMN d TYPE VARCHAR(64);