Fix IF NOT EXISTS in CREATE STATISTICS

I misplaced the IF NOT EXISTS clause in commit 7b504eb2, before the
word STATISTICS.  Put it where it belongs.

Patch written independently by Amit Langote and myself.  I adopted his
submitted test case with a slight edit also.

Reported-by: Bruno Wolff III
Discussion: https://postgr.es/m/20170621004237.GB8337@wolff.to

src/backend/parser/gram.y

@@ -3834,7 +3834,7 @@ ExistingIndex:   USING INDEX index_name				{ $$ = $3; }
 /*****************************************************************************
  *
  *		QUERY :
- *				CREATE STATISTICS stats_name [(stat types)]
+ *				CREATE STATISTICS [IF NOT EXISTS] stats_name [(stat types)]
  *					ON expression-list FROM from_list
  *
  * Note: the expectation here is that the clauses after ON are a subset of
@@ -3846,15 +3846,26 @@ ExistingIndex:   USING INDEX index_name				{ $$ = $3; }
  *****************************************************************************/
 
 CreateStatsStmt:
-			CREATE opt_if_not_exists STATISTICS any_name
+			CREATE STATISTICS any_name
 			opt_name_list ON expr_list FROM from_list
 				{
 					CreateStatsStmt *n = makeNode(CreateStatsStmt);
-					n->defnames = $4;
-					n->stat_types = $5;
-					n->exprs = $7;
-					n->relations = $9;
-					n->if_not_exists = $2;
+					n->defnames = $3;
+					n->stat_types = $4;
+					n->exprs = $6;
+					n->relations = $8;
+					n->if_not_exists = false;
+					$$ = (Node *)n;
+				}
+			| CREATE STATISTICS IF_P NOT EXISTS any_name
+			opt_name_list ON expr_list FROM from_list
+				{
+					CreateStatsStmt *n = makeNode(CreateStatsStmt);
+					n->defnames = $6;
+					n->stat_types = $7;
+					n->exprs = $9;
+					n->relations = $11;
+					n->if_not_exists = true;
 					$$ = (Node *)n;
 				}
 			;

src/test/regress/expected/stats_ext.out

@@ -30,9 +30,11 @@ CREATE STATISTICS tst ON (relpages, reltuples) FROM pg_class;
 ERROR:  only simple column references are allowed in CREATE STATISTICS
 CREATE STATISTICS tst (unrecognized) ON relname, relnatts FROM pg_class;
 ERROR:  unrecognized statistic type "unrecognized"
--- Ensure stats are dropped sanely
+-- Ensure stats are dropped sanely, and test IF NOT EXISTS while at it
 CREATE TABLE ab1 (a INTEGER, b INTEGER, c INTEGER);
-CREATE STATISTICS ab1_a_b_stats ON a, b FROM ab1;
+CREATE STATISTICS IF NOT EXISTS ab1_a_b_stats ON a, b FROM ab1;
+CREATE STATISTICS IF NOT EXISTS ab1_a_b_stats ON a, b FROM ab1;
+NOTICE:  statistics object "ab1_a_b_stats" already exists, skipping
 DROP STATISTICS ab1_a_b_stats;
 CREATE SCHEMA regress_schema_2;
 CREATE STATISTICS regress_schema_2.ab1_a_b_stats ON a, b FROM ab1;

src/test/regress/sql/stats_ext.sql

@@ -18,9 +18,10 @@ CREATE STATISTICS tst ON relnatts + relpages FROM pg_class;
 CREATE STATISTICS tst ON (relpages, reltuples) FROM pg_class;
 CREATE STATISTICS tst (unrecognized) ON relname, relnatts FROM pg_class;
 
--- Ensure stats are dropped sanely
+-- Ensure stats are dropped sanely, and test IF NOT EXISTS while at it
 CREATE TABLE ab1 (a INTEGER, b INTEGER, c INTEGER);
-CREATE STATISTICS ab1_a_b_stats ON a, b FROM ab1;
+CREATE STATISTICS IF NOT EXISTS ab1_a_b_stats ON a, b FROM ab1;
+CREATE STATISTICS IF NOT EXISTS ab1_a_b_stats ON a, b FROM ab1;
 DROP STATISTICS ab1_a_b_stats;
 
 CREATE SCHEMA regress_schema_2;