Pack tuples in a hash join batch densely, to save memory.

Instead of palloc'ing each HashJoinTuple individually, allocate 32kB chunks
and pack the tuples densely in the chunks. This avoids the AllocChunk
header overhead, and the space wasted by standard allocator's habit of
rounding sizes up to the nearest power of two.

This doesn't contain any planner changes, because the planner's estimate of
memory usage ignores the palloc overhead. Now that the overhead is smaller,
the planner's estimates are in fact more accurate.

Tomas Vondra, reviewed by Robert Haas.

src/backend/executor/nodeHash.c

@@ -47,6 +47,7 @@ static void ExecHashSkewTableInsert(HashJoinTable hashtable,
 						int bucketNumber);
 static void ExecHashRemoveNextSkewBucket(HashJoinTable hashtable);
 
+static void *dense_alloc(HashJoinTable hashtable, Size size);
 
 /* ----------------------------------------------------------------
  *		ExecHash
@@ -293,6 +294,7 @@ ExecHashTableCreate(Hash *node, List *hashOperators, bool keepNulls)
 	hashtable->spaceUsedSkew = 0;
 	hashtable->spaceAllowedSkew =
 		hashtable->spaceAllowed * SKEW_WORK_MEM_PERCENT / 100;
+	hashtable->chunks = NULL;
 
 	/*
 	 * Get info about the hash functions to be used for each hash key. Also
@@ -556,10 +558,10 @@ ExecHashIncreaseNumBatches(HashJoinTable hashtable)
 	int			oldnbatch = hashtable->nbatch;
 	int			curbatch = hashtable->curbatch;
 	int			nbatch;
-	int			i;
 	MemoryContext oldcxt;
 	long		ninmemory;
 	long		nfreed;
+	HashMemoryChunk oldchunks;
 
 	/* do nothing if we've decided to shut off growth */
 	if (!hashtable->growEnabled)
@@ -612,51 +614,65 @@ ExecHashIncreaseNumBatches(HashJoinTable hashtable)
 	 */
 	ninmemory = nfreed = 0;
 
-	for (i = 0; i < hashtable->nbuckets; i++)
-	{
-		HashJoinTuple prevtuple;
-		HashJoinTuple tuple;
+	/*
+	 * We will scan through the chunks directly, so that we can reset the
+	 * buckets now and not have to keep track which tuples in the buckets have
+	 * already been processed. We will free the old chunks as we go.
+	 */
+	memset(hashtable->buckets, 0, sizeof(HashJoinTuple *) * hashtable->nbuckets);
+	oldchunks = hashtable->chunks;
+	hashtable->chunks = NULL;
 
-		prevtuple = NULL;
-		tuple = hashtable->buckets[i];
+	/* so, let's scan through the old chunks, and all tuples in each chunk */
+	while (oldchunks != NULL)
+	{
+		HashMemoryChunk nextchunk = oldchunks->next;
+		/* position within the buffer (up to oldchunks->used) */
+		size_t		idx = 0;
 
-		while (tuple != NULL)
+		/* process all tuples stored in this chunk (and then free it) */
+		while (idx < oldchunks->used)
 		{
-			/* save link in case we delete */
-			HashJoinTuple nexttuple = tuple->next;
+			HashJoinTuple hashTuple = (HashJoinTuple) (oldchunks->data + idx);
+			MinimalTuple tuple = HJTUPLE_MINTUPLE(hashTuple);
+			int			hashTupleSize = (HJTUPLE_OVERHEAD + tuple->t_len);
 			int			bucketno;
 			int			batchno;
 
 			ninmemory++;
-			ExecHashGetBucketAndBatch(hashtable, tuple->hashvalue,
+			ExecHashGetBucketAndBatch(hashtable, hashTuple->hashvalue,
 									  &bucketno, &batchno);
-			Assert(bucketno == i);
+
 			if (batchno == curbatch)
 			{
-				/* keep tuple */
-				prevtuple = tuple;
+				/* keep tuple in memory - copy it into the new chunk */
+				HashJoinTuple copyTuple =
+					(HashJoinTuple) dense_alloc(hashtable, hashTupleSize);
+				memcpy(copyTuple, hashTuple, hashTupleSize);
+
+				/* and add it back to the appropriate bucket */
+				copyTuple->next = hashtable->buckets[bucketno];
+				hashtable->buckets[bucketno] = copyTuple;
 			}
 			else
 			{
 				/* dump it out */
 				Assert(batchno > curbatch);
-				ExecHashJoinSaveTuple(HJTUPLE_MINTUPLE(tuple),
-									  tuple->hashvalue,
+				ExecHashJoinSaveTuple(HJTUPLE_MINTUPLE(hashTuple),
+									  hashTuple->hashvalue,
 									  &hashtable->innerBatchFile[batchno]);
-				/* and remove from hash table */
-				if (prevtuple)
-					prevtuple->next = nexttuple;
-				else
-					hashtable->buckets[i] = nexttuple;
-				/* prevtuple doesn't change */
-				hashtable->spaceUsed -=
-					HJTUPLE_OVERHEAD + HJTUPLE_MINTUPLE(tuple)->t_len;
-				pfree(tuple);
+
+				hashtable->spaceUsed -= hashTupleSize;
 				nfreed++;
 			}
 
-			tuple = nexttuple;
+			/* next tuple in this chunk */
+			idx += MAXALIGN(hashTupleSize);
 		}
+
+		/* we're done with this chunk - free it and proceed to the next one */
+		pfree(oldchunks);
+		oldchunks = nextchunk;
 	}
 
 #ifdef HJDEBUG
@@ -717,8 +733,8 @@ ExecHashTableInsert(HashJoinTable hashtable,
 
 		/* Create the HashJoinTuple */
 		hashTupleSize = HJTUPLE_OVERHEAD + tuple->t_len;
-		hashTuple = (HashJoinTuple) MemoryContextAlloc(hashtable->batchCxt,
-													   hashTupleSize);
+		hashTuple = (HashJoinTuple) dense_alloc(hashtable, hashTupleSize);
+
 		hashTuple->hashvalue = hashvalue;
 		memcpy(HJTUPLE_MINTUPLE(hashTuple), tuple, tuple->t_len);
 
@@ -1068,6 +1084,9 @@ ExecHashTableReset(HashJoinTable hashtable)
 	hashtable->spaceUsed = 0;
 
 	MemoryContextSwitchTo(oldcxt);
+
+	/* Forget the chunks (the memory was freed by the context reset above). */
+	hashtable->chunks = NULL;
 }
 
 /*
@@ -1462,3 +1481,79 @@ ExecHashRemoveNextSkewBucket(HashJoinTable hashtable)
 		hashtable->spaceUsedSkew = 0;
 	}
 }
+
+/*
+ * Allocate 'size' bytes from the currently active HashMemoryChunk
+ */
+static void *
+dense_alloc(HashJoinTable hashtable, Size size)
+{
+	HashMemoryChunk newChunk;
+	char	   *ptr;
+
+	/* just in case the size is not already aligned properly */
+	size = MAXALIGN(size);
+
+	/*
+	 * If tuple size is larger than of 1/4 of chunk size, allocate a separate
+	 * chunk.
+	 */
+	if (size > HASH_CHUNK_THRESHOLD)
+	{
+		/* allocate new chunk and put it at the beginning of the list */
+		newChunk = (HashMemoryChunk) MemoryContextAlloc(hashtable->batchCxt,
+								  offsetof(HashMemoryChunkData, data) + size);
+		newChunk->maxlen = size;
+		newChunk->used = 0;
+		newChunk->ntuples = 0;
+
+		/*
+		 * Add this chunk to the list after the first existing chunk, so that
+		 * we don't lose the remaining space in the "current" chunk.
+		 */
+		if (hashtable->chunks != NULL)
+		{
+			newChunk->next = hashtable->chunks->next;
+			hashtable->chunks->next = newChunk;
+		}
+		else
+		{
+			newChunk->next = hashtable->chunks;
+			hashtable->chunks = newChunk;
+		}
+
+		newChunk->used += size;
+		newChunk->ntuples += 1;
+
+		return newChunk->data;
+	}
+
+	/*
+	 * See if we have enough space for it in the current chunk (if any).
+	 * If not, allocate a fresh chunk.
+	 */
+	if ((hashtable->chunks == NULL) ||
+		(hashtable->chunks->maxlen - hashtable->chunks->used) < size)
+	{
+		/* allocate new chunk and put it at the beginning of the list */
+		newChunk = (HashMemoryChunk) MemoryContextAlloc(hashtable->batchCxt,
+					   offsetof(HashMemoryChunkData, data) + HASH_CHUNK_SIZE);
+
+		newChunk->maxlen = HASH_CHUNK_SIZE;
+		newChunk->used = size;
+		newChunk->ntuples = 1;
+
+		newChunk->next = hashtable->chunks;
+		hashtable->chunks = newChunk;
+
+		return newChunk->data;
+	}
+
+	/* There is enough space in the current chunk, let's add the tuple */
+	ptr = hashtable->chunks->data + hashtable->chunks->used;
+	hashtable->chunks->used += size;
+	hashtable->chunks->ntuples += 1;
+
+	/* return pointer to the start of the tuple memory */
+	return ptr;
+}

src/include/executor/hashjoin.h

@@ -102,6 +102,25 @@ typedef struct HashSkewBucket
 #define SKEW_WORK_MEM_PERCENT  2
 #define SKEW_MIN_OUTER_FRACTION  0.01
 
+/*
+ * To reduce palloc overhead, the HashJoinTuples for the current batch are
+ * packed in 32kB buffers instead of pallocing each tuple individually.
+ */
+typedef struct HashMemoryChunkData
+{
+	int			ntuples;	/* number of tuples stored in this chunk */
+	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) */
+
+	char		data[1];	/* buffer allocated at the end */
+} HashMemoryChunkData;
+
+typedef struct HashMemoryChunkData *HashMemoryChunk;
+
+#define HASH_CHUNK_SIZE			(32 * 1024L)
+#define HASH_CHUNK_THRESHOLD	(HASH_CHUNK_SIZE / 4)
 
 typedef struct HashJoinTableData
 {
@@ -157,6 +176,9 @@ typedef struct HashJoinTableData
 
 	MemoryContext hashCxt;		/* context for whole-hash-join storage */
 	MemoryContext batchCxt;		/* context for this-batch-only storage */
+
+	/* used for dense allocation of tuples (into linked chunks) */
+	HashMemoryChunk chunks;		/*  one list for the whole batch */
 }	HashJoinTableData;
 
 #endif   /* HASHJOIN_H */