Prevent (auto)vacuum from truncating first page of populated matview.

Per report from Fujii Masao, with regression test using his example.

src/backend/commands/vacuumlazy.c

@@ -230,7 +230,13 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 	 *
 	 * Don't even think about it unless we have a shot at releasing a goodly
 	 * number of pages.  Otherwise, the time taken isn't worth it.
+	 *
+	 * Leave a populated materialized view with at least one page.
 	 */
+	if (onerel->rd_rel->relkind == RELKIND_MATVIEW &&
+		vacrelstats->nonempty_pages == 0)
+		vacrelstats->nonempty_pages = 1;
+
 	possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;
 	if (possibly_freeable > 0 &&
 		(possibly_freeable >= REL_TRUNCATE_MINIMUM ||

src/test/regress/expected/matview.out

@@ -418,3 +418,23 @@ NOTICE:  drop cascades to 3 other objects
 DETAIL:  drop cascades to view v_test2
 drop cascades to materialized view mv_test2
 drop cascades to materialized view mv_test3
+-- test that vacuum does not make empty matview look unpopulated
+CREATE TABLE hoge (i int);
+INSERT INTO hoge VALUES (generate_series(1,100000));
+CREATE MATERIALIZED VIEW hogeview AS SELECT * FROM hoge WHERE i % 2 = 0;
+CREATE INDEX hogeviewidx ON hogeview (i);
+DELETE FROM hoge;
+REFRESH MATERIALIZED VIEW hogeview;
+SELECT * FROM hogeview WHERE i < 10;
+ i 
+---
+(0 rows)
+
+VACUUM ANALYZE;
+SELECT * FROM hogeview WHERE i < 10;
+ i 
+---
+(0 rows)
+
+DROP TABLE hoge CASCADE;
+NOTICE:  drop cascades to materialized view hogeview

src/test/regress/sql/matview.sql

@@ -129,3 +129,15 @@ CREATE MATERIALIZED VIEW mv_test3 AS SELECT * FROM mv_test2 WHERE moo = 12345;
 SELECT pg_relation_is_scannable('mv_test3'::regclass);
 
 DROP VIEW v_test1 CASCADE;
+
+-- test that vacuum does not make empty matview look unpopulated
+CREATE TABLE hoge (i int);
+INSERT INTO hoge VALUES (generate_series(1,100000));
+CREATE MATERIALIZED VIEW hogeview AS SELECT * FROM hoge WHERE i % 2 = 0;
+CREATE INDEX hogeviewidx ON hogeview (i);
+DELETE FROM hoge;
+REFRESH MATERIALIZED VIEW hogeview;
+SELECT * FROM hogeview WHERE i < 10;
+VACUUM ANALYZE;
+SELECT * FROM hogeview WHERE i < 10;
+DROP TABLE hoge CASCADE;