Add parallel-aware hash joins.

Introduce parallel-aware hash joins that appear in EXPLAIN plans as Parallel
Hash Join with Parallel Hash.  While hash joins could already appear in
parallel queries, they were previously always parallel-oblivious and had a
partial subplan only on the outer side, meaning that the work of the inner
subplan was duplicated in every worker.

After this commit, the planner will consider using a partial subplan on the
inner side too, using the Parallel Hash node to divide the work over the
available CPU cores and combine its results in shared memory.  If the join
needs to be split into multiple batches in order to respect work_mem, then
workers process different batches as much as possible and then work together
on the remaining batches.

The advantages of a parallel-aware hash join over a parallel-oblivious hash
join used in a parallel query are that it:

 * avoids wasting memory on duplicated hash tables
 * avoids wasting disk space on duplicated batch files
 * divides the work of building the hash table over the CPUs

One disadvantage is that there is some communication between the participating
CPUs which might outweigh the benefits of parallelism in the case of small
hash tables.  This is avoided by the planner's existing reluctance to supply
partial plans for small scans, but it may be necessary to estimate
synchronization costs in future if that situation changes.  Another is that
outer batch 0 must be written to disk if multiple batches are required.

A potential future advantage of parallel-aware hash joins is that right and
full outer joins could be supported, since there is a single set of matched
bits for each hashtable, but that is not yet implemented.

A new GUC enable_parallel_hash is defined to control the feature, defaulting
to on.

Author: Thomas Munro
Reviewed-By: Andres Freund, Robert Haas
Tested-By: Rafia Sabih, Prabhat Sahu
Discussion:
    https://postgr.es/m/CAEepm=2W=cOkiZxcg6qiFQP-dHUe09aqTrEMM7yJDrHMhDv_RA@mail.gmail.com
    https://postgr.es/m/CAEepm=37HKyJ4U6XOLi=JgfSHM3o6B-GaeO-6hkOmneTDkH+Uw@mail.gmail.com

3 files changed, 198 insertions, 7 deletions

diff --git a/src/include/executor/hashjoin.h b/src/include/executor/hashjoin.h
index 82acadf85ba..d8c82d4e7c0 100644
--- a/src/include/executor/hashjoin.h
+++ b/src/include/executor/hashjoin.h
@@ -15,7 +15,10 @@
 #define HASHJOIN_H
 
 #include "nodes/execnodes.h"
+#include "port/atomics.h"
+#include "storage/barrier.h"
 #include "storage/buffile.h"
+#include "storage/lwlock.h"
 
 /* ----------------------------------------------------------------
  *				hash-join hash table structures
@@ -63,7 +66,12 @@
 
 typedef struct HashJoinTupleData
 {
-	struct HashJoinTupleData *next; /* link to next tuple in same bucket */
+	/* link to next tuple in same bucket */
+	union
+	{
+		struct HashJoinTupleData *unshared;
+		dsa_pointer shared;
+	}			next;
 	uint32		hashvalue;		/* tuple's hash code */
 	/* Tuple data, in MinimalTuple format, follows on a MAXALIGN boundary */
 }			HashJoinTupleData;
@@ -112,8 +120,12 @@ typedef struct HashMemoryChunkData
 	size_t		maxlen;			/* size of the buffer holding the tuples */
 	size_t		used;			/* number of buffer bytes already used */
 
-	struct HashMemoryChunkData *next;	/* pointer to the next chunk (linked
-										 * list) */
+	/* pointer to the next chunk (linked list) */
+	union
+	{
+		struct HashMemoryChunkData *unshared;
+		dsa_pointer shared;
+	}			next;
 
 	char		data[FLEXIBLE_ARRAY_MEMBER];	/* buffer allocated at the end */
 }			HashMemoryChunkData;
@@ -121,8 +133,148 @@ typedef struct HashMemoryChunkData
 typedef struct HashMemoryChunkData *HashMemoryChunk;
 
 #define HASH_CHUNK_SIZE			(32 * 1024L)
+#define HASH_CHUNK_HEADER_SIZE (offsetof(HashMemoryChunkData, data))
 #define HASH_CHUNK_THRESHOLD	(HASH_CHUNK_SIZE / 4)
 
+/*
+ * For each batch of a Parallel Hash Join, we have a ParallelHashJoinBatch
+ * object in shared memory to coordinate access to it.  Since they are
+ * followed by variable-sized objects, they are arranged in contiguous memory
+ * but not accessed directly as an array.
+ */
+typedef struct ParallelHashJoinBatch
+{
+	dsa_pointer buckets;		/* array of hash table buckets */
+	Barrier		batch_barrier;	/* synchronization for joining this batch */
+
+	dsa_pointer chunks;			/* chunks of tuples loaded */
+	size_t		size;			/* size of buckets + chunks in memory */
+	size_t		estimated_size; /* size of buckets + chunks while writing */
+	size_t		ntuples;		/* number of tuples loaded */
+	size_t		old_ntuples;	/* number of tuples before repartitioning */
+	bool		space_exhausted;
+
+	/*
+	 * Variable-sized SharedTuplestore objects follow this struct in memory.
+	 * See the accessor macros below.
+	 */
+} ParallelHashJoinBatch;
+
+/* Accessor for inner batch tuplestore following a ParallelHashJoinBatch. */
+#define ParallelHashJoinBatchInner(batch)							\
+	((SharedTuplestore *)											\
+	 ((char *) (batch) + MAXALIGN(sizeof(ParallelHashJoinBatch))))
+
+/* Accessor for outer batch tuplestore following a ParallelHashJoinBatch. */
+#define ParallelHashJoinBatchOuter(batch, nparticipants) \
+	((SharedTuplestore *)												\
+	 ((char *) ParallelHashJoinBatchInner(batch) +						\
+	  MAXALIGN(sts_estimate(nparticipants))))
+
+/* Total size of a ParallelHashJoinBatch and tuplestores. */
+#define EstimateParallelHashJoinBatch(hashtable)						\
+	(MAXALIGN(sizeof(ParallelHashJoinBatch)) +							\
+	 MAXALIGN(sts_estimate((hashtable)->parallel_state->nparticipants)) * 2)
+
+/* Accessor for the nth ParallelHashJoinBatch given the base. */
+#define NthParallelHashJoinBatch(base, n)								\
+	((ParallelHashJoinBatch *)											\
+	 ((char *) (base) +													\
+	  EstimateParallelHashJoinBatch(hashtable) *  (n)))
+
+/*
+ * Each backend requires a small amount of per-batch state to interact with
+ * each ParalellHashJoinBatch.
+ */
+typedef struct ParallelHashJoinBatchAccessor
+{
+	ParallelHashJoinBatch *shared;	/* pointer to shared state */
+
+	/* Per-backend partial counters to reduce contention. */
+	size_t		preallocated;	/* pre-allocated space for this backend */
+	size_t		ntuples;		/* number of tuples */
+	size_t		size;			/* size of partition in memory */
+	size_t		estimated_size; /* size of partition on disk */
+	size_t		old_ntuples;	/* how many tuples before repartioning? */
+	bool		at_least_one_chunk; /* has this backend allocated a chunk? */
+
+	bool		done;			/* flag to remember that a batch is done */
+	SharedTuplestoreAccessor *inner_tuples;
+	SharedTuplestoreAccessor *outer_tuples;
+} ParallelHashJoinBatchAccessor;
+
+/*
+ * While hashing the inner relation, any participant might determine that it's
+ * time to increase the number of buckets to reduce the load factor or batches
+ * to reduce the memory size.  This is indicated by setting the growth flag to
+ * these values.
+ */
+typedef enum ParallelHashGrowth
+{
+	/* The current dimensions are sufficient. */
+	PHJ_GROWTH_OK,
+	/* The load factor is too high, so we need to add buckets. */
+	PHJ_GROWTH_NEED_MORE_BUCKETS,
+	/* The memory budget would be exhausted, so we need to repartition. */
+	PHJ_GROWTH_NEED_MORE_BATCHES,
+	/* Repartitioning didn't help last time, so don't try to do that again. */
+	PHJ_GROWTH_DISABLED
+} ParallelHashGrowth;
+
+/*
+ * The shared state used to coordinate a Parallel Hash Join.  This is stored
+ * in the DSM segment.
+ */
+typedef struct ParallelHashJoinState
+{
+	dsa_pointer batches;		/* array of ParallelHashJoinBatch */
+	dsa_pointer old_batches;	/* previous generation during repartition */
+	int			nbatch;			/* number of batches now */
+	int			old_nbatch;		/* previous number of batches */
+	int			nbuckets;		/* number of buckets */
+	ParallelHashGrowth growth;	/* control batch/bucket growth */
+	dsa_pointer chunk_work_queue;	/* chunk work queue */
+	int			nparticipants;
+	size_t		space_allowed;
+	size_t		total_tuples;	/* total number of inner tuples */
+	LWLock		lock;			/* lock protecting the above */
+
+	Barrier		build_barrier;	/* synchronization for the build phases */
+	Barrier		grow_batches_barrier;
+	Barrier		grow_buckets_barrier;
+	pg_atomic_uint32 distributor;	/* counter for load balancing */
+
+	SharedFileSet fileset;		/* space for shared temporary files */
+} ParallelHashJoinState;
+
+/* The phases for building batches, used by build_barrier. */
+#define PHJ_BUILD_ELECTING				0
+#define PHJ_BUILD_ALLOCATING			1
+#define PHJ_BUILD_HASHING_INNER			2
+#define PHJ_BUILD_HASHING_OUTER			3
+#define PHJ_BUILD_DONE					4
+
+/* The phases for probing each batch, used by for batch_barrier. */
+#define PHJ_BATCH_ELECTING				0
+#define PHJ_BATCH_ALLOCATING			1
+#define PHJ_BATCH_LOADING				2
+#define PHJ_BATCH_PROBING				3
+#define PHJ_BATCH_DONE					4
+
+/* The phases of batch growth while hashing, for grow_batches_barrier. */
+#define PHJ_GROW_BATCHES_ELECTING		0
+#define PHJ_GROW_BATCHES_ALLOCATING		1
+#define PHJ_GROW_BATCHES_REPARTITIONING 2
+#define PHJ_GROW_BATCHES_DECIDING		3
+#define PHJ_GROW_BATCHES_FINISHING		4
+#define PHJ_GROW_BATCHES_PHASE(n)		((n) % 5)	/* circular phases */
+
+/* The phases of bucket growth while hashing, for grow_buckets_barrier. */
+#define PHJ_GROW_BUCKETS_ELECTING		0
+#define PHJ_GROW_BUCKETS_ALLOCATING		1
+#define PHJ_GROW_BUCKETS_REINSERTING	2
+#define PHJ_GROW_BUCKETS_PHASE(n)		((n) % 3)	/* circular phases */
+
 typedef struct HashJoinTableData
 {
 	int			nbuckets;		/* # buckets in the in-memory hash table */
@@ -133,8 +285,13 @@ typedef struct HashJoinTableData
 	int			log2_nbuckets_optimal;	/* log2(nbuckets_optimal) */
 
 	/* buckets[i] is head of list of tuples in i'th in-memory bucket */
-	struct HashJoinTupleData **buckets;
-	/* buckets array is per-batch storage, as are all the tuples */
+	union
+	{
+		/* unshared array is per-batch storage, as are all the tuples */
+		struct HashJoinTupleData **unshared;
+		/* shared array is per-query DSA area, as are all the tuples */
+		dsa_pointer_atomic *shared;
+	}			buckets;
 
 	bool		keepNulls;		/* true to store unmatchable NULL tuples */
 
@@ -153,6 +310,7 @@ typedef struct HashJoinTableData
 	bool		growEnabled;	/* flag to shut off nbatch increases */
 
 	double		totalTuples;	/* # tuples obtained from inner plan */
+	double		partialTuples;	/* # tuples obtained from inner plan by me */
 	double		skewTuples;		/* # tuples inserted into skew tuples */
 
 	/*
@@ -185,6 +343,13 @@ typedef struct HashJoinTableData
 
 	/* used for dense allocation of tuples (into linked chunks) */
 	HashMemoryChunk chunks;		/* one list for the whole batch */
+
+	/* Shared and private state for Parallel Hash. */
+	HashMemoryChunk current_chunk;	/* this backend's current chunk */
+	dsa_area   *area;			/* DSA area to allocate memory from */
+	ParallelHashJoinState *parallel_state;
+	ParallelHashJoinBatchAccessor *batches;
+	dsa_pointer current_chunk_shared;
 }			HashJoinTableData;
 
 #endif							/* HASHJOIN_H */
diff --git a/src/include/executor/nodeHash.h b/src/include/executor/nodeHash.h
index 0974f1edc21..84c166b3951 100644
--- a/src/include/executor/nodeHash.h
+++ b/src/include/executor/nodeHash.h
@@ -17,17 +17,33 @@
 #include "access/parallel.h"
 #include "nodes/execnodes.h"
 
+struct SharedHashJoinBatch;
+
 extern HashState *ExecInitHash(Hash *node, EState *estate, int eflags);
 extern Node *MultiExecHash(HashState *node);
 extern void ExecEndHash(HashState *node);
 extern void ExecReScanHash(HashState *node);
 
-extern HashJoinTable ExecHashTableCreate(Hash *node, List *hashOperators,
+extern HashJoinTable ExecHashTableCreate(HashState *state, List *hashOperators,
 					bool keepNulls);
+extern void ExecParallelHashTableAlloc(HashJoinTable hashtable,
+						   int batchno);
 extern void ExecHashTableDestroy(HashJoinTable hashtable);
+extern void ExecHashTableDetach(HashJoinTable hashtable);
+extern void ExecHashTableDetachBatch(HashJoinTable hashtable);
+extern void ExecParallelHashTableSetCurrentBatch(HashJoinTable hashtable,
+									 int batchno);
+void		ExecParallelHashUpdateSpacePeak(HashJoinTable hashtable, int batchno);
+
 extern void ExecHashTableInsert(HashJoinTable hashtable,
 					TupleTableSlot *slot,
 					uint32 hashvalue);
+extern void ExecParallelHashTableInsert(HashJoinTable hashtable,
+							TupleTableSlot *slot,
+							uint32 hashvalue);
+extern void ExecParallelHashTableInsertCurrentBatch(HashJoinTable hashtable,
+										TupleTableSlot *slot,
+										uint32 hashvalue);
 extern bool ExecHashGetHashValue(HashJoinTable hashtable,
 					 ExprContext *econtext,
 					 List *hashkeys,
@@ -39,12 +55,16 @@ extern void ExecHashGetBucketAndBatch(HashJoinTable hashtable,
 						  int *bucketno,
 						  int *batchno);
 extern bool ExecScanHashBucket(HashJoinState *hjstate, ExprContext *econtext);
+extern bool ExecParallelScanHashBucket(HashJoinState *hjstate, ExprContext *econtext);
 extern void ExecPrepHashTableForUnmatched(HashJoinState *hjstate);
 extern bool ExecScanHashTableForUnmatched(HashJoinState *hjstate,
 							  ExprContext *econtext);
 extern void ExecHashTableReset(HashJoinTable hashtable);
 extern void ExecHashTableResetMatchFlags(HashJoinTable hashtable);
 extern void ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
+						bool try_combined_work_mem,
+						int parallel_workers,
+						size_t *space_allowed,
 						int *numbuckets,
 						int *numbatches,
 						int *num_skew_mcvs);
@@ -55,6 +75,6 @@ extern void ExecHashInitializeWorker(HashState *node, ParallelWorkerContext *pwc
 extern void ExecHashRetrieveInstrumentation(HashState *node);
 extern void ExecShutdownHash(HashState *node);
 extern void ExecHashGetInstrumentation(HashInstrumentation *instrument,
-									   HashJoinTable hashtable);
+						   HashJoinTable hashtable);
 
 #endif							/* NODEHASH_H */
diff --git a/src/include/executor/nodeHashjoin.h b/src/include/executor/nodeHashjoin.h
index 7469bfbf60c..8469085d7e3 100644
--- a/src/include/executor/nodeHashjoin.h
+++ b/src/include/executor/nodeHashjoin.h
@@ -20,6 +20,12 @@
 extern HashJoinState *ExecInitHashJoin(HashJoin *node, EState *estate, int eflags);
 extern void ExecEndHashJoin(HashJoinState *node);
 extern void ExecReScanHashJoin(HashJoinState *node);
+extern void ExecShutdownHashJoin(HashJoinState *node);
+extern void ExecHashJoinEstimate(HashJoinState *state, ParallelContext *pcxt);
+extern void ExecHashJoinInitializeDSM(HashJoinState *state, ParallelContext *pcxt);
+extern void ExecHashJoinReInitializeDSM(HashJoinState *state, ParallelContext *pcxt);
+extern void ExecHashJoinInitializeWorker(HashJoinState *state,
+							 ParallelWorkerContext *pwcxt);
 
 extern void ExecHashJoinSaveTuple(MinimalTuple tuple, uint32 hashvalue,
 					  BufFile **fileptr);