PostgreSQL数据库查询——QueryRewrite

在pg_rewrite表中保存的是一个字符串形式的节点，通过stringtonode和nodetostring两个函数，将元数据表中的字符串转为相应的node类型的数据，这样就可以使用该node对象来替换query树中相应的节点。

src/backend/rewrite/rewriteHandler.c

QueryRewrite函数是重写器的主入口。
第一步，应用所有非select规则
第二步，对每个查询应用所有RIR规则
第三步，判定查询结果支持设置command-result标签，然后更新canSetTag域。如果原先的查询还在LIST中，设置command tag。否则，最后相同种类的INSTEAD查询被允许设置tag。

/* 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. */
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. */
  origCmdType = parsetree->commandType;
  foundOriginalQuery = false;
  lastInstead = NULL;

  foreach(l, results) {
    Query     *query = (Query *) lfirst(l);
    if (query->querySource == QSRC_ORIGINAL){
      Assert(query->canSetTag);
      Assert(!foundOriginalQuery);
      foundOriginalQuery = true;
#ifndef USE_ASSERT_CHECKING
      break;
#endif
    } else {
      Assert(!query->canSetTag);
      if (query->commandType == origCmdType && (query->querySource == QSRC_INSTEAD_RULE ||  query->querySource == QSRC_QUAL_INSTEAD_RULE))
        lastInstead = query;
    }
  }

  if (!foundOriginalQuery && lastInstead != NULL)
    lastInstead->canSetTag = true;
  return results;
}