Function CheckFieldCoverage

pg_documentdb/src/opclass/index_support.c:2134–2264

tree_walker_callback that recursively checks if all field paths in the tree are covered by the index. * Returns true to exit early if any uncovered path is found. */

/* tree_walker_callback that recursively checks if all field paths in the tree are covered by the index.
 * Returns true to exit early if any uncovered path is found. */
static bool
CheckFieldCoverage(Node *node, void *context)
{
	check_stack_depth();
	CHECK_FOR_INTERRUPTS();

	FieldCoverageState *state = (FieldCoverageState *) context;

	if (node == NULL)
	{
		return false;
	}

	/* We need to check Aggrefs in addition to functions because they may have a field path directly under the Aggref, not inside a FuncExpr. */
	if (IsA(node, Aggref))
	{
		/* There are two shapes of aggregates:
		 *  1. The child node is a FuncExpr
		 *  2. The field path is directly on the Aggref args
		 *
		 *  We try to find a direct Var(document) and a BSON const.
		 *  If we can't find it, we are probably in case one, so we recurse to the FuncExpr.
		 *  If we find it, we check if the field path is covered by the index. If not, we can mark the field coverage as uncovered and abort.
		 */

		const char *fieldPath = NULL;
		bool sawDocumentVar = false;

		Aggref *aggref = (Aggref *) node;
		ListCell *aggArgCell;

		foreach(aggArgCell, aggref->args)
		{
			Expr *argExpr = StripRelabels(((TargetEntry *) lfirst(aggArgCell))->expr);
			if (IsA(argExpr, Var) &&
				IsCurrentScanDocumentVar((Var *) argExpr, state->root,
										 state->expectedRti))
			{
				sawDocumentVar = true;
				continue;
			}

			/* Currently, we only handle aggregate functions with two arguments, and one field path. */
			if (fieldPath == NULL)
			{
				fieldPath = TryExtractFieldPathFromConst(argExpr);
			}
		}

		if (sawDocumentVar && fieldPath != NULL)
		{
			if (!IsFieldPathCoveredByIndex(fieldPath, state->indexPath))
			{
				/* Field path is not in this index so we can't do index-only. */
				state->hasUncoveredField = true;
				return true; /* abort the walk early */
			}