Fix unexpected reversal of lists during catcache rehash

During catcache searches, the most-recently searched entries are kept at
the head of the list to speed up subsequent searches, keeping the
"freshest" entries at its beginning.  A rehash of the catcache was doing
the opposite: fresh entries were moved to the tail of the newly-created
buckets, causing a rehash to slow down a bit.

When a rehash is done, this commit switches the code to use
dlist_push_tail() instead of dlist_push_head(), so as fresh entries are
kept at the head of the lists, not their tail.

Author: ChangAo Chen <cca5507@qq.com>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/tencent_9EA10D8512B5FE29E7323F780A0749768708@qq.com

diff --git a/src/backend/utils/cache/catcache.c b/src/backend/utils/cache/catcache.c
index 9cfda91a877ce03d65203b06b8d254c994d488ae..681aa923403f0af10a7db1af9f0897f6da792648 100644 (file)
--- a/src/backend/utils/cache/catcache.c
+++ b/src/backend/utils/cache/catcache.c
@@ -1013,7 +1013,14 @@ RehashCatCache(CatCache *cp)
                        int                     hashIndex = HASH_INDEX(ct->hash_value, newnbuckets);
 
                        dlist_delete(iter.cur);
-                       dlist_push_head(&newbucket[hashIndex], &ct->cache_elem);
+
+                       /*
+                        * Note that each item is pushed at the tail of the new bucket,
+                        * not its head.  This is consistent with the SearchCatCache*()
+                        * routines, where matching entries are moved at the front of the
+                        * list to speed subsequent searches.
+                        */
+                       dlist_push_tail(&newbucket[hashIndex], &ct->cache_elem);
                }
        }
 
@@ -1051,7 +1058,14 @@ RehashCatCacheLists(CatCache *cp)
                        int                     hashIndex = HASH_INDEX(cl->hash_value, newnbuckets);
 
                        dlist_delete(iter.cur);
-                       dlist_push_head(&newbucket[hashIndex], &cl->cache_elem);
+
+                       /*
+                        * Note that each item is pushed at the tail of the new bucket,
+                        * not its head.  This is consistent with the SearchCatCache*()
+                        * routines, where matching entries are moved at the front of the
+                        * list to speed subsequent searches.
+                        */
+                       dlist_push_tail(&newbucket[hashIndex], &cl->cache_elem);
                }
        }