Function tdigest_combine

pg_documentdb/src/aggregation/bson_tdigest.c:1076–1144

Datum
tdigest_combine(PG_FUNCTION_ARGS)
{
	tdigest_aggstate_t *src;
	tdigest_aggstate_t *dst;
	MemoryContext aggcontext;
	MemoryContext oldcontext;

	if (!AggCheckCallContext(fcinfo, &aggcontext))
	{
		elog(ERROR, "tdigest_combine called in non-aggregate context");
	}

	/* if no "merged" state yet, try creating it */
	if (PG_ARGISNULL(0))
	{
		/* nope, the second argument is NULL to, so return NULL */
		if (PG_ARGISNULL(1))
		{
			PG_RETURN_NULL();
		}

		/* the second argument is not NULL, so copy it */
		src = (tdigest_aggstate_t *) PG_GETARG_POINTER(1);

		/* copy the digest into the right long-lived memory context */
		oldcontext = MemoryContextSwitchTo(aggcontext);
		src = tdigest_copy(src);
		MemoryContextSwitchTo(oldcontext);

		PG_RETURN_POINTER(src);
	}

	/*
	 * If the second argument is NULL, just return the first one (we know
	 * it's not NULL at this point).
	 */
	if (PG_ARGISNULL(1))
	{
		PG_RETURN_DATUM(PG_GETARG_DATUM(0));
	}

	/* Now we know neither argument is NULL, so merge them. */
	src = (tdigest_aggstate_t *) PG_GETARG_POINTER(1);
	dst = (tdigest_aggstate_t *) PG_GETARG_POINTER(0);

	/*
	 * Do a compaction on each digest, to make sure we have enough space.
	 *
	 * XXX Maybe do this only when necessary, i.e. when we can't fit the
	 * data into the dst digest? Also, is it really ensured this gives us
	 * enough free space?
	 */
	tdigest_compact(dst);
	tdigest_compact(src);

	/* copy the second part */
	memcpy(&dst->centroids[dst->ncentroids],