Improve test coverage for functional dependencies

The regression tests for functional dependencies were only using clauses
of the form (Var op Const), i.e. with Var on the left side. This adds
a couple of queries with Var on the right, to test other code paths.

It also prints one of the functional dependencies, to test the data type
output function. The functional dependencies are "perfect" with degree
of 1.0 so this should be stable.

Discussion: https://www.postgresql.org/message-id/flat/20200113230008.g67iyk4cs3xbnjju@development
Author: Tomas Vondra

src/test/regress/expected/stats_ext.out

@@ -536,6 +536,13 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE
 -- create statistics
 CREATE STATISTICS func_deps_stat (dependencies) ON a, b, c FROM functional_dependencies;
 ANALYZE functional_dependencies;
+-- print the detected dependencies
+SELECT dependencies FROM pg_stats_ext WHERE statistics_name = 'func_deps_stat';
+                                                dependencies                                                
+------------------------------------------------------------------------------------------------------------
+ {"3 => 4": 1.000000, "3 => 6": 1.000000, "4 => 6": 1.000000, "3, 4 => 6": 1.000000, "3, 6 => 4": 1.000000}
+(1 row)
+
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a = 1 AND b = ''1''');
  estimated | actual 
 -----------+--------
@@ -697,6 +704,12 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi
        102 |    714
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND 0 = b');
+ estimated | actual 
+-----------+--------
+       102 |    714
+(1 row)
+
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE c = 0 AND d = 0');
  estimated | actual 
 -----------+--------
@@ -709,6 +722,12 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi
          1 |     64
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND b = 0 AND 0 = c AND d = 0');
+ estimated | actual 
+-----------+--------
+         1 |     64
+(1 row)
+
 -- create separate functional dependencies
 CREATE STATISTICS functional_dependencies_multi_1 (dependencies) ON a, b FROM functional_dependencies_multi;
 CREATE STATISTICS functional_dependencies_multi_2 (dependencies) ON c, d FROM functional_dependencies_multi;
@@ -719,6 +738,12 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi
        714 |    714
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND 0 = b');
+ estimated | actual 
+-----------+--------
+       714 |    714
+(1 row)
+
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE c = 0 AND d = 0');
  estimated | actual 
 -----------+--------
@@ -731,6 +756,12 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi
         65 |     64
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND b = 0 AND 0 = c AND d = 0');
+ estimated | actual 
+-----------+--------
+        65 |     64
+(1 row)
+
 DROP TABLE functional_dependencies_multi;
 -- MCV lists
 CREATE TABLE mcv_lists (

src/test/regress/sql/stats_ext.sql

@@ -318,6 +318,9 @@ CREATE STATISTICS func_deps_stat (dependencies) ON a, b, c FROM functional_depen
 
 ANALYZE functional_dependencies;
 
+-- print the detected dependencies
+SELECT dependencies FROM pg_stats_ext WHERE statistics_name = 'func_deps_stat';
+
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a = 1 AND b = ''1''');
 
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a = 1 AND b = ''1'' AND c = 1');
@@ -391,8 +394,10 @@ ANALYZE functional_dependencies_multi;
 
 -- estimates without any functional dependencies
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE a = 0 AND b = 0');
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND 0 = b');
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE c = 0 AND d = 0');
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE a = 0 AND b = 0 AND c = 0 AND d = 0');
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND b = 0 AND 0 = c AND d = 0');
 
 -- create separate functional dependencies
 CREATE STATISTICS functional_dependencies_multi_1 (dependencies) ON a, b FROM functional_dependencies_multi;
@@ -401,8 +406,10 @@ CREATE STATISTICS functional_dependencies_multi_2 (dependencies) ON c, d FROM fu
 ANALYZE functional_dependencies_multi;
 
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE a = 0 AND b = 0');
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND 0 = b');
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE c = 0 AND d = 0');
 SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE a = 0 AND b = 0 AND c = 0 AND d = 0');
+SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies_multi WHERE 0 = a AND b = 0 AND 0 = c AND d = 0');
 
 DROP TABLE functional_dependencies_multi;