本文共 2389 字，大约阅读时间需要 7 分钟。
接前面，回溯调用关系：
exec_simple_query --> PortalStart --> ExecutorStart --> StandardExecutorStart --> InitPlan
再回到 exec_simple_query 来：
事前知道，表 tst04 对应的文件名为 16393。
postgres=# select oid from pg_class where relname='tst04'; oid ------- 16393(1 row)postgres=#
看 exec_simple_query，加点调试信息：
static voidexec_simple_query(const char *query_string){ ... parsetree_list = pg_parse_query(query_string); ... /* * Run through the raw parsetree(s) and process each one. */ foreach(parsetree_item, parsetree_list) { ... querytree_list = pg_analyze_and_rewrite(parsetree, query_string,NULL, 0); plantree_list = pg_plan_queries(querytree_list, 0, NULL); ... portal = CreatePortal("", true, true); ... PortalDefineQuery(portal, NULL, query_string, commandTag, plantree_list, NULL); ... PortalStart(portal, NULL, 0, snapshot_set); ... fprintf(stderr,"Before we call PortalRun,Now sleep for 120 seconds!!!\n"); sleep(120); /* * Run the portal to completion, and then drop it (and the receiver). */ (void) PortalRun(portal, FETCH_ALL, isTopLevel, receiver, receiver, completionTag); fprintf(stderr,"After we call PortalRun!!!\n"); ... } /* end loop over parsetrees */ ...} 在PortalRun之前，让它休眠120秒，在此期间，将 tst04表对应文件改名。
测试发现，改名对执行无影响、在同一客户端，再次发起对同名表的sql问，没有问题：
[postgres@lex pgsql]$ ./bin/pg_ctl -D ./data startserver starting[postgres@lex pgsql]$ LOG: database system was shut down at 2013-05-30 13:15:52 CSTLOG: autovacuum launcher startedLOG: database system is ready to accept connectionsBefore we call PortalRun,Now sleep for 120 seconds!!!After we call PortalRun!!!Before we call PortalRun,Now sleep for 120 seconds!!!After we call PortalRun!!!
[postgres@lex 12788]$ mv 16393 16393.bak[postgres@lex 12788]$
postgres=# select val from tst04; val ----- 100 200 300(3 rows)postgres=# select id from tst04; id ---- 1 2 3(3 rows)postgres=#
这说明：一旦准备好了执行计划，对文件的打开句柄将保持（除非因lru之类算法被移除）。
本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2013/05/30/3108018.html，如需转载请自行联系原作者