Fix outdated comments, GIST search queue is not an RBTree anymore.

The GiST search queue is implemented as a pairing heap rather than as
Red-Black Tree, since 9.5 (commit e7032610). I neglected these comments
in that commit.

diff --git a/src/backend/access/gist/gistscan.c b/src/backend/access/gist/gistscan.c
index ba611ee490a93b9ee2b5346a3b40cffe07ef0c65..2526a3965c421fc88a3b8c3ca866be7b07a671c0 100644 (file)
--- a/src/backend/access/gist/gistscan.c
+++ b/src/backend/access/gist/gistscan.c
@@ -125,7 +125,7 @@ gistrescan(IndexScanDesc scan, ScanKey key, int nkeys,
         * which is created on the second call and reset on later calls.  Thus, in
         * the common case where a scan is only rescan'd once, we just put the
         * queue in scanCxt and don't pay the overhead of making a second memory
-        * context.  If we do rescan more than once, the first RBTree is just left
+        * context.  If we do rescan more than once, the first queue is just left
         * for dead until end of scan; this small wastage seems worth the savings
         * in the common case.
         */
@@ -181,7 +181,7 @@ gistrescan(IndexScanDesc scan, ScanKey key, int nkeys,
                                                                                                ALLOCSET_DEFAULT_SIZES);
        }
 
-       /* create new, empty RBTree for search queue */
+       /* create new, empty pairing heap for search queue */
        oldCxt = MemoryContextSwitchTo(so->queueCxt);
        so->queue = pairingheap_allocate(pairingheap_GISTSearchItem_cmp, scan);
        MemoryContextSwitchTo(oldCxt);

diff --git a/src/include/access/gist_private.h b/src/include/access/gist_private.h
index 123158501731a355eadd5e049bf7bab23c450f7c..78e87a607718549747e9b97e1e473d6508948894 100644 (file)
--- a/src/include/access/gist_private.h
+++ b/src/include/access/gist_private.h
@@ -107,15 +107,11 @@ typedef struct GISTSTATE
  * upper index pages; this rule avoids doing extra work during a search that
  * ends early due to LIMIT.
  *
- * To perform an ordered search, we use an RBTree to manage the distance-order
- * queue.  Each GISTSearchTreeItem stores all unvisited items of the same
- * distance; they are GISTSearchItems chained together via their next fields.
- *
- * In a non-ordered search (no order-by operators), the RBTree degenerates
- * to a single item, which we use as a queue of unvisited index pages only.
- * In this case matched heap items from the current index leaf page are
- * remembered in GISTScanOpaqueData.pageData[] and returned directly from
- * there, instead of building a separate GISTSearchItem for each one.
+ * To perform an ordered search, we use a pairing heap to manage the
+ * distance-order queue.  In a non-ordered search (no order-by operators),
+ * we use it to return heap tuples before unvisited index pages, to
+ * ensure depth-first order, but all entries are otherwise considered
+ * equal.
  */
 
 /* Individual heap tuple to be visited */
@@ -298,8 +294,8 @@ typedef struct
 #define GIST_ROOT_BLKNO                                0
 
 /*
- * Before PostgreSQL 9.1, we used rely on so-called "invalid tuples" on inner
- * pages to finish crash recovery of incomplete page splits. If a crash
+ * Before PostgreSQL 9.1, we used to rely on so-called "invalid tuples" on
+ * inner pages to finish crash recovery of incomplete page splits. If a crash
  * happened in the middle of a page split, so that the downlink pointers were
  * not yet inserted, crash recovery inserted a special downlink pointer. The
  * semantics of an invalid tuple was that it if you encounter one in a scan,