Fix a couple of problems in pg_get_statisticsextdef

There was a thinko whereby we tested the wrong tuple after fetching it
from cache; avoid that by using generate_relation_name instead, which is
simpler.  Also, the statistics name was not qualified, so add that.  (It
could be argued that qualification should be conditional on the schema
not being on search path.  We can add that later, but at least this form
is correct.)

Author: David Rowley, Álvaro Herrera
Discussion: https://postgr.es/m/CAKJS1f8RjLeVZJ2+93pdQGuZJeBF-ifsHaFMR-q-6-Z0qxA8cA@mail.gmail.com

src/backend/utils/adt/ruleutils.c

@@ -1448,11 +1448,10 @@ static char *
 pg_get_statisticsext_worker(Oid statextid, bool missing_ok)
 {
 	Form_pg_statistic_ext	statextrec;
-	Form_pg_class			pgclassrec;
 	HeapTuple	statexttup;
-	HeapTuple	pgclasstup;
 	StringInfoData buf;
 	int			colno;
+	char	   *nsp;
 
 	statexttup = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(statextid));
 
@@ -1465,20 +1464,12 @@ pg_get_statisticsext_worker(Oid statextid, bool missing_ok)
 
 	statextrec = (Form_pg_statistic_ext) GETSTRUCT(statexttup);
 
-	pgclasstup = SearchSysCache1(RELOID, ObjectIdGetDatum(statextrec->starelid));
-
-	if (!HeapTupleIsValid(statexttup))
-	{
-		ReleaseSysCache(statexttup);
-		elog(ERROR, "cache lookup failed for relation %u", statextrec->starelid);
-	}
-
-	pgclassrec = (Form_pg_class) GETSTRUCT(pgclasstup);
-
 	initStringInfo(&buf);
 
+	nsp = get_namespace_name(statextrec->stanamespace);
 	appendStringInfo(&buf, "CREATE STATISTICS %s ON (",
-							quote_identifier(NameStr(statextrec->staname)));
+					 quote_qualified_identifier(nsp,
+												NameStr(statextrec->staname)));
 
 	for (colno = 0; colno < statextrec->stakeys.dim1; colno++)
 	{
@@ -1494,10 +1485,9 @@ pg_get_statisticsext_worker(Oid statextid, bool missing_ok)
 	}
 
 	appendStringInfo(&buf, ") FROM %s",
-							quote_identifier(NameStr(pgclassrec->relname)));
+					 generate_relation_name(statextrec->starelid, NIL));
 
 	ReleaseSysCache(statexttup);
-	ReleaseSysCache(pgclasstup);
 
 	return buf.data;
 }

src/test/regress/expected/rules.out

@@ -3167,6 +3167,12 @@ SELECT pg_get_ruledef(0);
  
 (1 row)
 
+SELECT pg_get_statisticsextdef(0);
+ pg_get_statisticsextdef 
+-------------------------
+ 
+(1 row)
+
 SELECT pg_get_triggerdef(0);
  pg_get_triggerdef 
 -------------------

src/test/regress/expected/stats_ext.out

@@ -5,6 +5,13 @@ CREATE STATISTICS ab1_a_b_stats ON (a, b) FROM ab1;
 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;
+-- Let's also verify the pg_get_statisticsextdef output looks sane.
+SELECT pg_get_statisticsextdef(oid) FROM pg_statistic_ext WHERE staname = 'ab1_a_b_stats';
+                       pg_get_statisticsextdef                       
+---------------------------------------------------------------------
+ CREATE STATISTICS regress_schema_2.ab1_a_b_stats ON (a, b) FROM ab1
+(1 row)
+
 DROP STATISTICS regress_schema_2.ab1_a_b_stats;
 -- Ensure statistics are dropped when columns are
 CREATE STATISTICS ab1_b_c_stats ON (b, c) FROM ab1;

src/test/regress/sql/rules.sql

@@ -1150,6 +1150,7 @@ SELECT pg_get_constraintdef(0);
 SELECT pg_get_functiondef(0);
 SELECT pg_get_indexdef(0);
 SELECT pg_get_ruledef(0);
+SELECT pg_get_statisticsextdef(0);
 SELECT pg_get_triggerdef(0);
 SELECT pg_get_viewdef(0);
 SELECT pg_get_function_arguments(0);

src/test/regress/sql/stats_ext.sql

@@ -7,6 +7,10 @@ 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;
+
+-- Let's also verify the pg_get_statisticsextdef output looks sane.
+SELECT pg_get_statisticsextdef(oid) FROM pg_statistic_ext WHERE staname = 'ab1_a_b_stats';
+
 DROP STATISTICS regress_schema_2.ab1_a_b_stats;
 
 -- Ensure statistics are dropped when columns are