Function foreign_expr_walker

contrib/postgres_fdw/deparse.c:299–867

* Check if expression is safe to execute remotely, and return true if so. * * In addition, *outer_cxt is updated with collation information. * * We must check that the expression contains only node types we can deparse, * that all types/functions/operators are safe to send (they are "shippable"), * and that all collations used in the expression derive from Vars of the * foreign table. Beca

 * currently considered here.
 */
static bool
foreign_expr_walker(Node *node,
					foreign_glob_cxt *glob_cxt,
					foreign_loc_cxt *outer_cxt)
{
	bool		check_type = true;
	PgFdwRelationInfo *fpinfo;
	foreign_loc_cxt inner_cxt;
	Oid			collation;
	FDWCollateState state;

	/* Need do nothing for empty subexpressions */
	if (node == NULL)
		return true;

	/* May need server info from baserel's fdw_private struct */
	fpinfo = (PgFdwRelationInfo *) (glob_cxt->foreignrel->fdw_private);

	/* Set up inner_cxt for possible recursion to child nodes */
	inner_cxt.collation = InvalidOid;
	inner_cxt.state = FDW_COLLATE_NONE;

	switch (nodeTag(node))
	{
		case T_Var:
			{
				Var		   *var = (Var *) node;

				/*
				 * If the Var is from the foreign table, we consider its
				 * collation (if any) safe to use.  If it is from another
				 * table, we treat its collation the same way as we would a
				 * Param's collation, ie it's not safe for it to have a
				 * non-default collation.
				 */
				if (bms_is_member(var->varno, glob_cxt->relids) &&
					var->varlevelsup == 0)
				{
					/* Var belongs to foreign table */

					/*
					 * System columns other than ctid should not be sent to
					 * the remote, since we don't make any effort to ensure
					 * that local and remote values match (tableoid, in
					 * particular, almost certainly doesn't match).
					 */
					if (var->varattno < 0 &&
						var->varattno != SelfItemPointerAttributeNumber)
						return false;

					/* Else check the collation */
					collation = var->varcollid;
					state = OidIsValid(collation) ? FDW_COLLATE_SAFE : FDW_COLLATE_NONE;
				}
				else
				{
					/* Var belongs to some other table */
					collation = var->varcollid;