Merge pull request #4630 from citusdata/fix_4626

Columnar: Fix zero column tables

src/backend/columnar/cstore_reader.c

@@ -21,6 +21,7 @@
 #include "access/nbtree.h"
 #include "catalog/pg_am.h"
 #include "commands/defrem.h"
+#include "distributed/listutils.h"
 #include "nodes/makefuncs.h"
 #if PG_VERSION_NUM >= 120000
 #include "nodes/nodeFuncs.h"
@@ -46,6 +47,13 @@ struct TableReadState
 	TupleDesc tupleDescriptor;
 	Relation relation;
 
+	/*
+	 * Following are used for tables with zero columns, or when no
+	 * columns are projected.
+	 */
+	uint64 totalRowCount;
+	uint64 readRowCount;
+
 	/*
 	 * List of Var pointers for columns in the query. We use this both for
 	 * getting vector of projected columns, and also when we want to build
@@ -112,6 +120,13 @@ ColumnarBeginRead(Relation relation, TupleDesc tupleDescriptor,
 				  List *projectedColumnList, List *whereClauseList)
 {
 	List *stripeList = StripesForRelfilenode(relation->rd_node);
+	StripeMetadata *stripeMetadata = NULL;
+
+	uint64 totalRowCount = 0;
+	foreach_ptr(stripeMetadata, stripeList)
+	{
+		totalRowCount += stripeMetadata->rowCount;
+	}
 
 	/*
 	 * We allocate all stripe specific data in the stripeReadContext, and reset
@@ -135,6 +150,8 @@ ColumnarBeginRead(Relation relation, TupleDesc tupleDescriptor,
 	readState->stripeReadContext = stripeReadContext;
 	readState->chunkData = NULL;
 	readState->deserializedChunkIndex = -1;
+	readState->readRowCount = 0;
+	readState->totalRowCount = totalRowCount;
 
 	return readState;
 }
@@ -151,6 +168,26 @@ ColumnarReadNextRow(TableReadState *readState, Datum *columnValues, bool *column
 	StripeMetadata *stripeMetadata = readState->currentStripeMetadata;
 	MemoryContext oldContext = NULL;
 
+	/*
+	 * We rely on first column's metadata in rest of this function. So for zero
+	 * column tables we just return "true" for totalRowCount times. We do the
+	 * same when no columns are projected.
+	 */
+	if (readState->projectedColumnList == NIL)
+	{
+		if (readState->totalRowCount == readState->readRowCount)
+		{
+			return false;
+		}
+		else
+		{
+			int columnCount = readState->tupleDescriptor->natts;
+			memset(columnNulls, 1, sizeof(bool) * columnCount);
+			readState->readRowCount++;
+			return true;
+		}
+	}
+
 	/*
 	 * If no stripes are loaded, load the next non-empty stripe. Note that when
 	 * loading stripes, we skip over chunks whose contents can be filtered with

src/test/regress/expected/am_alter.out

@@ -240,5 +240,99 @@ SELECT * FROM test_gen_ex;
  3 | 4
 (3 rows)
 
+-- check removing all columns while having some data to simulate
+-- table with non-zero rows but zero-columns.
+-- https://github.com/citusdata/citus/issues/4626
+BEGIN;
+create table local(y int);
+insert into local values (1), (2);
+alter table local drop column y;
+CREATE TABLE zero_col_columnar (like local) USING COLUMNAR;
+ALTER TABLE local RENAME TO local_xxxxx;
+INSERT INTO zero_col_columnar SELECT * FROM local_xxxxx;
+COMMIT;
+SELECT * FROM zero_col_columnar;
+--
+(2 rows)
+
+SELECT count(*) FROM zero_col_columnar;
+ count
+---------------------------------------------------------------------
+     2
+(1 row)
+
+EXPLAIN (costs off, summary off) SELECT * FROM zero_col_columnar;
+                   QUERY PLAN
+---------------------------------------------------------------------
+ Custom Scan (ColumnarScan) on zero_col_columnar
+(1 row)
+
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+SELECT * FROM zero_col_columnar;
+--
+(5 rows)
+
+SELECT count(*) FROM zero_col_columnar;
+ count
+---------------------------------------------------------------------
+     5
+(1 row)
+
+EXPLAIN (costs off, summary off) SELECT * FROM zero_col_columnar;
+                   QUERY PLAN
+---------------------------------------------------------------------
+ Custom Scan (ColumnarScan) on zero_col_columnar
+(1 row)
+
+VACUUM VERBOSE zero_col_columnar;
+INFO:  statistics for "zero_col_columnar":
+storage id: xxxxx
+total file size: 16384, total data size: 0
+compression rate: 1.00x
+total row count: 5, stripe count: 4, average rows per stripe: 1
+chunk count: 0, containing data for dropped columns: 0
+
+ANALYZE zero_col_columnar;
+VACUUM FULL zero_col_columnar;
+SELECT * FROM zero_col_columnar;
+--
+(5 rows)
+
+TRUNCATE zero_col_columnar;
+SELECT * FROM zero_col_columnar;
+--
+(0 rows)
+
+DROP TABLE zero_col_columnar;
+CREATE TABLE zero_col_columnar(a int) USING columnar;
+INSERT INTO zero_col_columnar SELECT i FROM generate_series(1, 5) i;
+alter table zero_col_columnar drop column a;
+SELECT * FROM zero_col_columnar;
+--
+(5 rows)
+
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+SELECT * FROM zero_col_columnar;
+--
+(8 rows)
+
+VACUUM VERBOSE zero_col_columnar;
+INFO:  statistics for "zero_col_columnar":
+storage id: xxxxx
+total file size: 49152, total data size: 60
+compression rate: 0.40x
+total row count: 8, stripe count: 4, average rows per stripe: 2
+chunk count: 4, containing data for dropped columns: 4, zstd compressed: 4
+
+ANALYZE zero_col_columnar;
+VACUUM FULL zero_col_columnar;
+SELECT * FROM zero_col_columnar;
+--
+(8 rows)
+
 SET client_min_messages TO WARNING;
 DROP SCHEMA columnar_alter CASCADE;

src/test/regress/sql/am_alter.sql

@@ -123,5 +123,60 @@ INSERT INTO test_gen_ex VALUES (1), (2), (3);
 ALTER TABLE test_gen_ex ADD COLUMN y int generated always as (x+1) stored;
 SELECT * FROM test_gen_ex;
 
+
+-- check removing all columns while having some data to simulate
+-- table with non-zero rows but zero-columns.
+-- https://github.com/citusdata/citus/issues/4626
+BEGIN;
+create table local(y int);
+insert into local values (1), (2);
+alter table local drop column y;
+
+CREATE TABLE zero_col_columnar (like local) USING COLUMNAR;
+ALTER TABLE local RENAME TO local_xxxxx;
+INSERT INTO zero_col_columnar SELECT * FROM local_xxxxx;
+COMMIT;
+
+SELECT * FROM zero_col_columnar;
+SELECT count(*) FROM zero_col_columnar;
+EXPLAIN (costs off, summary off) SELECT * FROM zero_col_columnar;
+
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+SELECT * FROM zero_col_columnar;
+SELECT count(*) FROM zero_col_columnar;
+EXPLAIN (costs off, summary off) SELECT * FROM zero_col_columnar;
+
+VACUUM VERBOSE zero_col_columnar;
+ANALYZE zero_col_columnar;
+VACUUM FULL zero_col_columnar;
+
+SELECT * FROM zero_col_columnar;
+
+TRUNCATE zero_col_columnar;
+
+SELECT * FROM zero_col_columnar;
+
+DROP TABLE zero_col_columnar;
+
+CREATE TABLE zero_col_columnar(a int) USING columnar;
+INSERT INTO zero_col_columnar SELECT i FROM generate_series(1, 5) i;
+alter table zero_col_columnar drop column a;
+
+SELECT * FROM zero_col_columnar;
+
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+INSERT INTO zero_col_columnar DEFAULT VALUES;
+
+SELECT * FROM zero_col_columnar;
+
+VACUUM VERBOSE zero_col_columnar;
+ANALYZE zero_col_columnar;
+VACUUM FULL zero_col_columnar;
+
+SELECT * FROM zero_col_columnar;
+
 SET client_min_messages TO WARNING;
 DROP SCHEMA columnar_alter CASCADE;