Hashed crosstab was dying with an SPI_finish error when the source SQL

produced no rows. Now it returns 0 rows instead. Adjusted regression
test for this case.

contrib/tablefunc/expected/tablefunc.out

@@ -127,7 +127,7 @@ SELECT * FROM crosstab('SELECT rowid, attribute, val FROM ct where rowclass = ''
 -- hash based crosstab
 --
 create table cth(id serial, rowid text, rowdt timestamp, attribute text, val text);
-NOTICE:  CREATE TABLE will create implicit sequence "cth_id_seq" for "serial" column "cth.id"
+NOTICE:  CREATE TABLE will create implicit sequence "cth_id_seq" for serial column "cth.id"
 insert into cth values(DEFAULT,'test1','01 March 2003','temperature','42');
 insert into cth values(DEFAULT,'test1','01 March 2003','test_result','PASS');
 -- the next line is intentionally left commented and is therefore a "missing" attribute
@@ -193,6 +193,24 @@ SELECT * FROM crosstab(
   'SELECT DISTINCT rowdt, attribute FROM cth ORDER BY 2')
 AS c(rowid text, rowdt timestamp, temperature int4, test_result text, test_startdate timestamp, volts float8);
 ERROR:  provided "categories" SQL must return 1 column of at least one row
+-- if source query returns zero rows, get zero rows returned
+SELECT * FROM crosstab(
+  'SELECT rowid, rowdt, attribute, val FROM cth WHERE false ORDER BY 1',
+  'SELECT DISTINCT attribute FROM cth ORDER BY 1')
+AS c(rowid text, rowdt timestamp, temperature text, test_result text, test_startdate text, volts text);
+ rowid | rowdt | temperature | test_result | test_startdate | volts 
+-------+-------+-------------+-------------+----------------+-------
+(0 rows)
+
+-- if source query returns zero rows, get zero rows returned even if category query generates no rows
+SELECT * FROM crosstab(
+  'SELECT rowid, rowdt, attribute, val FROM cth WHERE false ORDER BY 1',
+  'SELECT DISTINCT attribute FROM cth WHERE false ORDER BY 1')
+AS c(rowid text, rowdt timestamp, temperature text, test_result text, test_startdate text, volts text);
+ rowid | rowdt | temperature | test_result | test_startdate | volts 
+-------+-------+-------------+-------------+----------------+-------
+(0 rows)
+
 --
 -- connectby
 --

contrib/tablefunc/sql/tablefunc.sql

@@ -88,6 +88,17 @@ SELECT * FROM crosstab(
   'SELECT DISTINCT rowdt, attribute FROM cth ORDER BY 2')
 AS c(rowid text, rowdt timestamp, temperature int4, test_result text, test_startdate timestamp, volts float8);
 
+-- if source query returns zero rows, get zero rows returned
+SELECT * FROM crosstab(
+  'SELECT rowid, rowdt, attribute, val FROM cth WHERE false ORDER BY 1',
+  'SELECT DISTINCT attribute FROM cth ORDER BY 1')
+AS c(rowid text, rowdt timestamp, temperature text, test_result text, test_startdate text, volts text);
+
+-- if source query returns zero rows, get zero rows returned even if category query generates no rows
+SELECT * FROM crosstab(
+  'SELECT rowid, rowdt, attribute, val FROM cth WHERE false ORDER BY 1',
+  'SELECT DISTINCT attribute FROM cth WHERE false ORDER BY 1')
+AS c(rowid text, rowdt timestamp, temperature text, test_result text, test_startdate text, volts text);
 
 --
 -- connectby

contrib/tablefunc/tablefunc.c

@@ -821,15 +821,6 @@ load_categories_hash(char *cats_sql, MemoryContext per_query_ctx)
 			MemoryContextSwitchTo(SPIcontext);
 		}
 	}
-	else
-	{
-		/* no qualifying tuples */
-		SPI_finish();
-		ereport(ERROR,
-				(errcode(ERRCODE_SYNTAX_ERROR),
-				 errmsg("provided \"categories\" SQL must " \
-						"return 1 column of at least one row")));
-	}
 
 	if (SPI_finish() != SPI_OK_FINISH)
 		/* internal error */
@@ -879,6 +870,15 @@ get_crosstab_tuplestore(char *sql,
 					j;
 		int			result_ncols;
 
+		if (num_categories == 0)
+		{
+			/* no qualifying category tuples */
+			ereport(ERROR,
+					(errcode(ERRCODE_SYNTAX_ERROR),
+					errmsg("provided \"categories\" SQL must " \
+							"return 1 column of at least one row")));
+		}
+
 		/*
 		 * The provided SQL query must always return at least three
 		 * columns:
@@ -994,11 +994,6 @@ get_crosstab_tuplestore(char *sql,
 		MemoryContextSwitchTo(SPIcontext);
 
 	}
-	else
-	{
-		/* no qualifying tuples */
-		SPI_finish();
-	}
 
 	if (SPI_finish() != SPI_OK_FINISH)
 		/* internal error */