Implement Incremental Sort

Incremental Sort is an optimized variant of multikey sort for cases when
the input is already sorted by a prefix of the requested sort keys. For
example when the relation is already sorted by (key1, key2) and we need
to sort it by (key1, key2, key3) we can simply split the input rows into
groups having equal values in (key1, key2), and only sort/compare the
remaining column key3.

This has a number of benefits:

- Reduced memory consumption, because only a single group (determined by
  values in the sorted prefix) needs to be kept in memory. This may also
  eliminate the need to spill to disk.

- Lower startup cost, because Incremental Sort produce results after each
  prefix group, which is beneficial for plans where startup cost matters
  (like for example queries with LIMIT clause).

We consider both Sort and Incremental Sort, and decide based on costing.

The implemented algorithm operates in two different modes:

- Fetching a minimum number of tuples without check of equality on the
  prefix keys, and sorting on all columns when safe.

- Fetching all tuples for a single prefix group and then sorting by
  comparing only the remaining (non-prefix) keys.

We always start in the first mode, and employ a heuristic to switch into
the second mode if we believe it's beneficial - the goal is to minimize
the number of unnecessary comparions while keeping memory consumption
below work_mem.

This is a very old patch series. The idea was originally proposed by
Alexander Korotkov back in 2013, and then revived in 2017. In 2018 the
patch was taken over by James Coleman, who wrote and rewrote most of the
current code.

There were many reviewers/contributors since 2013 - I've done my best to
pick the most active ones, and listed them in this commit message.

Author: James Coleman, Alexander Korotkov
Reviewed-by: Tomas Vondra, Andreas Karlsson, Marti Raudsepp, Peter Geoghegan, Robert Haas, Thomas Munro, Antonin Houska, Andres Freund, Alexander Kuzmenkov
Discussion: https://postgr.es/m/CAPpHfdscOX5an71nHd8WSUH6GNOCf=V7wgDaTXdDd9=goN-gfA@mail.gmail.com
Discussion: https://postgr.es/m/CAPpHfds1waRZ=NOmueYq0sx1ZSCnt+5QJvizT8ndT2=etZEeAQ@mail.gmail.com

src/include/executor/execdebug.h

@@ -86,10 +86,12 @@
 #define SO_nodeDisplay(l)				nodeDisplay(l)
 #define SO_printf(s)					printf(s)
 #define SO1_printf(s, p)				printf(s, p)
+#define SO2_printf(s, p1, p2)			printf(s, p1, p2)
 #else
 #define SO_nodeDisplay(l)
 #define SO_printf(s)
 #define SO1_printf(s, p)
+#define SO2_printf(s, p1, p2)
 #endif							/* EXEC_SORTDEBUG */
 
 /* ----------------

src/include/executor/nodeIncrementalSort.h

@@ -0,0 +1,28 @@
+/*-------------------------------------------------------------------------
+ *
+ * nodeIncrementalSort.h
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/executor/nodeIncrementalSort.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef NODEINCREMENTALSORT_H
+#define NODEINCREMENTALSORT_H
+
+#include "access/parallel.h"
+#include "nodes/execnodes.h"
+
+extern IncrementalSortState *ExecInitIncrementalSort(IncrementalSort *node, EState *estate, int eflags);
+extern void ExecEndIncrementalSort(IncrementalSortState *node);
+extern void ExecReScanIncrementalSort(IncrementalSortState *node);
+
+/* parallel instrumentation support */
+extern void ExecIncrementalSortEstimate(IncrementalSortState *node, ParallelContext *pcxt);
+extern void ExecIncrementalSortInitializeDSM(IncrementalSortState *node, ParallelContext *pcxt);
+extern void ExecIncrementalSortInitializeWorker(IncrementalSortState *node, ParallelWorkerContext *pcxt);
+extern void ExecIncrementalSortRetrieveInstrumentation(IncrementalSortState *node);
+
+#endif							/* NODEINCREMENTALSORT_H */

src/include/nodes/execnodes.h

@@ -1982,6 +1982,21 @@ typedef struct MaterialState
 	Tuplestorestate *tuplestorestate;
 } MaterialState;
 
+
+/* ----------------
+ *	 When performing sorting by multiple keys, it's possible that the input
+ *	 dataset is already sorted on a prefix of those keys. We call these
+ *	 "presorted keys".
+ *	 PresortedKeyData represents information about one such key.
+ * ----------------
+ */
+typedef struct PresortedKeyData
+{
+	FmgrInfo	flinfo;			/* comparison function info */
+	FunctionCallInfo fcinfo;	/* comparison function call info */
+	OffsetNumber attno;			/* attribute number in tuple */
+} PresortedKeyData;
+
 /* ----------------
  *	 Shared memory container for per-worker sort information
  * ----------------
@@ -2010,6 +2025,71 @@ typedef struct SortState
 	SharedSortInfo *shared_info;	/* one entry per worker */
 } SortState;
 
+/* ----------------
+ *	 Instrumentation information for IncrementalSort
+ * ----------------
+ */
+typedef struct IncrementalSortGroupInfo
+{
+	int64		groupCount;
+	long		maxDiskSpaceUsed;
+	long		totalDiskSpaceUsed;
+	long		maxMemorySpaceUsed;
+	long		totalMemorySpaceUsed;
+	bits32		sortMethods; /* bitmask of TuplesortMethod */
+} IncrementalSortGroupInfo;
+
+typedef struct IncrementalSortInfo
+{
+	IncrementalSortGroupInfo fullsortGroupInfo;
+	IncrementalSortGroupInfo prefixsortGroupInfo;
+} IncrementalSortInfo;
+
+/* ----------------
+ *	 Shared memory container for per-worker incremental sort information
+ * ----------------
+ */
+typedef struct SharedIncrementalSortInfo
+{
+	int			num_workers;
+	IncrementalSortInfo sinfo[FLEXIBLE_ARRAY_MEMBER];
+} SharedIncrementalSortInfo;
+
+/* ----------------
+ *	 IncrementalSortState information
+ * ----------------
+ */
+typedef enum
+{
+	INCSORT_LOADFULLSORT,
+	INCSORT_LOADPREFIXSORT,
+	INCSORT_READFULLSORT,
+	INCSORT_READPREFIXSORT,
+} IncrementalSortExecutionStatus;
+
+typedef struct IncrementalSortState
+{
+	ScanState	ss;				/* its first field is NodeTag */
+	bool		bounded;		/* is the result set bounded? */
+	int64		bound;			/* if bounded, how many tuples are needed */
+	bool		outerNodeDone;	/* finished fetching tuples from outer node */
+	int64		bound_Done;		/* value of bound we did the sort with */
+	IncrementalSortExecutionStatus execution_status;
+	int64		n_fullsort_remaining;
+	Tuplesortstate *fullsort_state; /* private state of tuplesort.c */
+	Tuplesortstate *prefixsort_state;	/* private state of tuplesort.c */
+	/* the keys by which the input path is already sorted */
+	PresortedKeyData *presorted_keys;
+
+	IncrementalSortInfo incsort_info;
+
+	/* slot for pivot tuple defining values of presorted keys within group */
+	TupleTableSlot *group_pivot;
+	TupleTableSlot *transfer_tuple;
+	bool		am_worker;		/* are we a worker? */
+	SharedIncrementalSortInfo *shared_info; /* one entry per worker */
+} IncrementalSortState;
+
 /* ---------------------
  *	GroupState information
  * ---------------------

src/include/nodes/nodes.h

@@ -74,6 +74,7 @@ typedef enum NodeTag
 	T_HashJoin,
 	T_Material,
 	T_Sort,
+	T_IncrementalSort,
 	T_Group,
 	T_Agg,
 	T_WindowAgg,
@@ -130,6 +131,7 @@ typedef enum NodeTag
 	T_HashJoinState,
 	T_MaterialState,
 	T_SortState,
+	T_IncrementalSortState,
 	T_GroupState,
 	T_AggState,
 	T_WindowAggState,
@@ -245,6 +247,7 @@ typedef enum NodeTag
 	T_ProjectionPath,
 	T_ProjectSetPath,
 	T_SortPath,
+	T_IncrementalSortPath,
 	T_GroupPath,
 	T_UpperUniquePath,
 	T_AggPath,

src/include/nodes/pathnodes.h

@@ -1638,6 +1638,15 @@ typedef struct SortPath
 	Path	   *subpath;		/* path representing input source */
 } SortPath;
 
+/*
+ * IncrementalSortPath
+ */
+typedef struct IncrementalSortPath
+{
+	SortPath	spath;
+	int			nPresortedCols;	/* number of presorted columns */
+} IncrementalSortPath;
+
 /*
  * GroupPath represents grouping (of presorted input)
  *

src/include/nodes/plannodes.h

@@ -774,6 +774,16 @@ typedef struct Sort
 	bool	   *nullsFirst;		/* NULLS FIRST/LAST directions */
 } Sort;
 
+/* ----------------
+ *		incremental sort node
+ * ----------------
+ */
+typedef struct IncrementalSort
+{
+	Sort		sort;
+	int			nPresortedCols;	/* number of presorted columns */
+} IncrementalSort;
+
 /* ---------------
  *	 group node -
  *		Used for queries with GROUP BY (but no aggregates) specified.

src/include/optimizer/cost.h

@@ -53,6 +53,7 @@ extern PGDLLIMPORT bool enable_indexonlyscan;
 extern PGDLLIMPORT bool enable_bitmapscan;
 extern PGDLLIMPORT bool enable_tidscan;
 extern PGDLLIMPORT bool enable_sort;
+extern PGDLLIMPORT bool enable_incrementalsort;
 extern PGDLLIMPORT bool enable_hashagg;
 extern PGDLLIMPORT bool enable_hashagg_disk;
 extern PGDLLIMPORT bool enable_groupingsets_hash_disk;
@@ -103,6 +104,11 @@ extern void cost_sort(Path *path, PlannerInfo *root,
 					  List *pathkeys, Cost input_cost, double tuples, int width,
 					  Cost comparison_cost, int sort_mem,
 					  double limit_tuples);
+extern void cost_incremental_sort(Path *path,
+								  PlannerInfo *root, List *pathkeys, int presorted_keys,
+								  Cost input_startup_cost, Cost input_total_cost,
+								  double input_tuples, int width, Cost comparison_cost, int sort_mem,
+								  double limit_tuples);
 extern void cost_append(AppendPath *path);
 extern void cost_merge_append(Path *path, PlannerInfo *root,
 							  List *pathkeys, int n_streams,

src/include/optimizer/pathnode.h

@@ -184,6 +184,12 @@ extern ProjectSetPath *create_set_projection_path(PlannerInfo *root,
 												  RelOptInfo *rel,
 												  Path *subpath,
 												  PathTarget *target);
+extern SortPath *create_incremental_sort_path(PlannerInfo *root,
+											  RelOptInfo *rel,
+											  Path *subpath,
+											  List *pathkeys,
+											  int presorted_keys,
+											  double limit_tuples);
 extern SortPath *create_sort_path(PlannerInfo *root,
 								  RelOptInfo *rel,
 								  Path *subpath,

src/include/optimizer/paths.h

@@ -185,6 +185,7 @@ typedef enum
 
 extern PathKeysComparison compare_pathkeys(List *keys1, List *keys2);
 extern bool pathkeys_contained_in(List *keys1, List *keys2);
+extern bool pathkeys_count_contained_in(List *keys1, List *keys2, int *n_common);
 extern Path *get_cheapest_path_for_pathkeys(List *paths, List *pathkeys,
 											Relids required_outer,
 											CostSelector cost_criterion,

src/include/utils/tuplesort.h

@@ -61,14 +61,17 @@ typedef struct SortCoordinateData *SortCoordinate;
  * Data structures for reporting sort statistics.  Note that
  * TuplesortInstrumentation can't contain any pointers because we
  * sometimes put it in shared memory.
+ *
+ * TuplesortMethod is used in a bitmask in Increment Sort's shared memory
+ * instrumentation so needs to have each value be a separate bit.
  */
 typedef enum
 {
-	SORT_TYPE_STILL_IN_PROGRESS = 0,
-	SORT_TYPE_TOP_N_HEAPSORT,
-	SORT_TYPE_QUICKSORT,
-	SORT_TYPE_EXTERNAL_SORT,
-	SORT_TYPE_EXTERNAL_MERGE
+	SORT_TYPE_STILL_IN_PROGRESS = 1 << 0,
+	SORT_TYPE_TOP_N_HEAPSORT = 1 << 1,
+	SORT_TYPE_QUICKSORT = 1 << 2,
+	SORT_TYPE_EXTERNAL_SORT = 1 << 3,
+	SORT_TYPE_EXTERNAL_MERGE = 1 << 4
 } TuplesortMethod;
 
 typedef enum
@@ -215,6 +218,7 @@ extern Tuplesortstate *tuplesort_begin_datum(Oid datumType,
 											 bool randomAccess);
 
 extern void tuplesort_set_bound(Tuplesortstate *state, int64 bound);
+extern bool tuplesort_used_bound(Tuplesortstate *state);
 
 extern void tuplesort_puttupleslot(Tuplesortstate *state,
 								   TupleTableSlot *slot);
@@ -239,6 +243,8 @@ extern bool tuplesort_skiptuples(Tuplesortstate *state, int64 ntuples,
 
 extern void tuplesort_end(Tuplesortstate *state);
 
+extern void tuplesort_reset(Tuplesortstate *state);
+
 extern void tuplesort_get_stats(Tuplesortstate *state,
 								TuplesortInstrumentation *stats);
 extern const char *tuplesort_method_name(TuplesortMethod m);