Function dense_alloc

src/backend/executor/nodeHash.c:3424–3491

* Allocate 'size' bytes from the currently active HashMemoryChunk */

 * 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 threshold, 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,
														HASH_CHUNK_HEADER_SIZE + size);
		newChunk->maxlen = size;
		newChunk->used = size;
		newChunk->ntuples = 1;

		/*
		 * 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.unshared = newChunk;
		}
		else
		{
			newChunk->next.unshared = hashtable->chunks;
			hashtable->chunks = newChunk;
		}

		return HASH_CHUNK_DATA(newChunk);
	}

	/*
	 * 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,
														HASH_CHUNK_HEADER_SIZE + HASH_CHUNK_SIZE);

		newChunk->maxlen = HASH_CHUNK_SIZE;
		newChunk->used = size;
		newChunk->ntuples = 1;

		newChunk->next.unshared = hashtable->chunks;
		hashtable->chunks = newChunk;

		return HASH_CHUNK_DATA(newChunk);