Handle better the hint table if the extension is not created

An environment enabling pg_hint_plan.enable_hint_table while
pg_hint_plan has not been created as an extension would fail all its
queries as the table created by the extension does not exist.

This commit adds a check at the top of get_hints_from_table() to check
if the extension is installed before attempting to use the hint table,
generating a WARNING.  We have discussed about a few options, but a
WARNING is more consistent with how duplicated hints or compute_query_id
disabled are handled.

This does not completely close the failure hole, though, as it is still
possible to see the table lookup failure for the CREATE EXTENSION
command enabling pg_hint_plan as an extension if enable_hint_table is
enabled.  In terms of code simplicity, I am not really convinced that
we need to do more just for that.

This commit relies on 490f869d92e5 that has introduced syscache entries
for pg_extension.  On stable branches, we will need a different logic
with a check on the table itself with its namespace.

Idea based on some feedback from Julien Rouhaud.  The tests are from me.

Author: Sami Imseih, Michael Paquier

Backpatch-through: 12

Per issue #191.

Makefile

@@ -13,7 +13,7 @@ OBJS = \
 HINTPLANVER = 1.7.0
 
 REGRESS = init base_plan pg_hint_plan ut-init ut-A ut-S ut-J ut-L ut-G ut-R \
-	ut-fdw ut-W ut-T ut-fini plpgsql oldextversions
+	ut-fdw ut-W ut-T ut-fini plpgsql hint_table oldextversions
 REGRESS_OPTS = --encoding=UTF8
 
 EXTENSION = pg_hint_plan

expected/hint_table.out

@@ -0,0 +1,24 @@
+-- Tests for the hint table
+LOAD 'pg_hint_plan';
+-- Attempting to use the hint table without the extension created
+-- emits a WARNING.
+SET pg_hint_plan.enable_hint_table TO on;
+SELECT 1;
+WARNING:  cannot use the hint table
+HINT:  Run "CREATE EXTENSION pg_hint_plan" to create the hint table.
+ ?column? 
+----------
+        1
+(1 row)
+
+SET pg_hint_plan.enable_hint_table TO off;
+CREATE EXTENSION pg_hint_plan;
+SET pg_hint_plan.enable_hint_table TO on;
+SELECT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+SET pg_hint_plan.enable_hint_table TO off;
+DROP EXTENSION pg_hint_plan;

pg_hint_plan.c

@@ -13,6 +13,7 @@
 #include "access/genam.h"
 #include "access/heapam.h"
 #include "access/relation.h"
+#include "catalog/namespace.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_index.h"
 #include "catalog/pg_proc.h"
@@ -1856,6 +1857,25 @@ get_hints_from_table(uint64 queryId, const char *client_application)
 	Datum	values[2];
 	char 	nulls[2] = {' ', ' '};
 	text   *app;
+	Oid		namespaceId;
+	bool	hints_table_found = false;
+
+	/*
+	 * Make sure that hint_plan.hints is found before we attempt to look for
+	 * a hint.
+	 */
+	namespaceId = LookupExplicitNamespace("hint_plan", true);
+	if (OidIsValid(namespaceId) &&
+		OidIsValid(get_relname_relid("hints", namespaceId)))
+		hints_table_found = true;
+
+	if (!hints_table_found)
+	{
+		ereport(WARNING,
+				(errmsg ("cannot use the hint table"),
+				 errhint("Run \"CREATE EXTENSION pg_hint_plan\" to create the hint table.")));
+		return NULL;
+	}
 
 	PG_TRY();
 	{

sql/hint_table.sql

@@ -0,0 +1,14 @@
+-- Tests for the hint table
+LOAD 'pg_hint_plan';
+
+-- Attempting to use the hint table without the extension created
+-- emits a WARNING.
+SET pg_hint_plan.enable_hint_table TO on;
+SELECT 1;
+SET pg_hint_plan.enable_hint_table TO off;
+
+CREATE EXTENSION pg_hint_plan;
+SET pg_hint_plan.enable_hint_table TO on;
+SELECT 1;
+SET pg_hint_plan.enable_hint_table TO off;
+DROP EXTENSION pg_hint_plan;