Function QueryRewrite

src/backend/rewrite/rewriteHandler.c:4186–4276

* QueryRewrite - * Primary entry point to the query rewriter. * Rewrite one query via query rewrite system, possibly returning 0 * or many queries. * * NOTE: the parsetree must either have come straight from the parser, * or have been scanned by AcquireRewriteLocks to acquire suitable locks. */

 * or have been scanned by AcquireRewriteLocks to acquire suitable locks.
 */
List *
QueryRewrite(Query *parsetree)
{
	uint64		input_query_id = parsetree->queryId;
	List	   *querylist;
	List	   *results;
	ListCell   *l;
	CmdType		origCmdType;
	bool		foundOriginalQuery;
	Query	   *lastInstead;

	/*
	 * This function is only applied to top-level original queries
	 */
	Assert(parsetree->querySource == QSRC_ORIGINAL);
	Assert(parsetree->canSetTag);

	/*
	 * Step 1
	 *
	 * Apply all non-SELECT rules possibly getting 0 or many queries
	 */
	querylist = RewriteQuery(parsetree, NIL);

	/*
	 * Step 2
	 *
	 * Apply all the RIR rules on each query
	 *
	 * This is also a handy place to mark each query with the original queryId
	 */
	results = NIL;
	foreach(l, querylist)
	{
		Query	   *query = (Query *) lfirst(l);

		query = fireRIRrules(query, NIL);

		query->queryId = input_query_id;

		results = lappend(results, query);
	}

	/*
	 * Step 3
	 *
	 * Determine which, if any, of the resulting queries is supposed to set
	 * the command-result tag; and update the canSetTag fields accordingly.
	 *
	 * If the original query is still in the list, it sets the command tag.
	 * Otherwise, the last INSTEAD query of the same kind as the original is
	 * allowed to set the tag.  (Note these rules can leave us with no query
	 * setting the tag.  The tcop code has to cope with this by setting up a
	 * default tag based on the original un-rewritten query.)
	 *
	 * The Asserts verify that at most one query in the result list is marked
	 * canSetTag.  If we aren't checking asserts, we can fall out of the loop
	 * as soon as we find the original query.