Optimize the case where a btree indexscan has current and mark positions

on the same index page; we can avoid data copying as well as buffer refcount
manipulations in this common case.  Makes for a small but noticeable
improvement in mergejoin speed.

Heikki Linnakangas

diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index 3329321c0ffd14727301296d441ace0dd1ad4bce..c58974cca139fabec66eb6fcf521d77de697cb1b 100644 (file)
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.149 2006/05/10 23:18:39 tgl Exp $
+ *   $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.150 2006/08/24 01:18:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -392,6 +392,7 @@ btrescan(PG_FUNCTION_ARGS)
        ReleaseBuffer(so->markPos.buf);
        so->markPos.buf = InvalidBuffer;
    }
+   so->markItemIndex = -1;
 
    /*
     * Reset the scan keys. Note that keys ordering stuff moved to _bt_first.
@@ -430,6 +431,7 @@ btendscan(PG_FUNCTION_ARGS)
        ReleaseBuffer(so->markPos.buf);
        so->markPos.buf = InvalidBuffer;
    }
+   so->markItemIndex = -1;
 
    if (so->killedItems != NULL)
        pfree(so->killedItems);
@@ -456,14 +458,16 @@ btmarkpos(PG_FUNCTION_ARGS)
        so->markPos.buf = InvalidBuffer;
    }
 
-   /* bump pin on current buffer for assignment to mark buffer */
+   /*
+    * Just record the current itemIndex.  If we later step to next page
+    * before releasing the marked position, _bt_steppage makes a full copy
+    * of the currPos struct in markPos.  If (as often happens) the mark is
+    * moved before we leave the page, we don't have to do that work.
+    */
    if (BTScanPosIsValid(so->currPos))
-   {
-       IncrBufferRefCount(so->currPos.buf);
-       memcpy(&so->markPos, &so->currPos,
-              offsetof(BTScanPosData, items[1]) +
-              so->currPos.lastItem * sizeof(BTScanPosItem));
-   }
+       so->markItemIndex = so->currPos.itemIndex;
+   else
+       so->markItemIndex = -1;
 
    PG_RETURN_VOID();
 }
@@ -477,24 +481,35 @@ btrestrpos(PG_FUNCTION_ARGS)
    IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
-   /* we aren't holding any read locks, but gotta drop the pin */
-   if (BTScanPosIsValid(so->currPos))
+   if (so->markItemIndex >= 0)
    {
-       /* Before leaving current page, deal with any killed items */
-       if (so->numKilled > 0 &&
-           so->currPos.buf != so->markPos.buf)
-           _bt_killitems(scan, false);
-       ReleaseBuffer(so->currPos.buf);
-       so->currPos.buf = InvalidBuffer;
-   }
-
-   /* bump pin on marked buffer */
-   if (BTScanPosIsValid(so->markPos))
+       /*
+        * The mark position is on the same page we are currently on.
+        * Just restore the itemIndex.
+        */
+       so->currPos.itemIndex = so->markItemIndex;
+   } 
+   else
    {
-       IncrBufferRefCount(so->markPos.buf);
-       memcpy(&so->currPos, &so->markPos,
-              offsetof(BTScanPosData, items[1]) +
-              so->markPos.lastItem * sizeof(BTScanPosItem));
+       /* we aren't holding any read locks, but gotta drop the pin */
+       if (BTScanPosIsValid(so->currPos))
+       {
+           /* Before leaving current page, deal with any killed items */
+           if (so->numKilled > 0 &&
+               so->currPos.buf != so->markPos.buf)
+               _bt_killitems(scan, false);
+           ReleaseBuffer(so->currPos.buf);
+           so->currPos.buf = InvalidBuffer;
+       }
+
+       if (BTScanPosIsValid(so->markPos))
+       {
+           /* bump pin on mark buffer for assignment to current buffer */
+           IncrBufferRefCount(so->markPos.buf);
+           memcpy(&so->currPos, &so->markPos,
+                  offsetof(BTScanPosData, items[1]) +
+                  so->markPos.lastItem * sizeof(BTScanPosItem));
+       }
    }
 
    PG_RETURN_VOID();

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 2c1dfc3eb474ffef4ab801f146532c9e2aa500b7..07bc076e49b3026cf30cc0f38c3b7eadbb905549 100644 (file)
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.105 2006/05/07 01:21:30 tgl Exp $
+ *   $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.106 2006/08/24 01:18:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -815,6 +815,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
        so->currPos.moreRight = false;
    }
    so->numKilled = 0;          /* just paranoia */
+   so->markItemIndex = -1;     /* ditto */
 
    /* position to the precise item on the page */
    offnum = _bt_binsrch(rel, buf, keysCount, scankeys, nextkey);
@@ -1053,6 +1054,21 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
    if (so->numKilled > 0)
        _bt_killitems(scan, true);
 
+   /*
+    * Before we modify currPos, make a copy of the page data if there
+    * was a mark position that needs it.
+    */
+   if (so->markItemIndex >= 0)
+   {
+       /* bump pin on current buffer for assignment to mark buffer */
+       IncrBufferRefCount(so->currPos.buf);
+       memcpy(&so->markPos, &so->currPos,
+              offsetof(BTScanPosData, items[1]) +
+              so->currPos.lastItem * sizeof(BTScanPosItem));
+       so->markPos.itemIndex = so->markItemIndex;
+       so->markItemIndex = -1;
+   }
+
    rel = scan->indexRelation;
 
    if (ScanDirectionIsForward(dir))
@@ -1408,6 +1424,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
        so->currPos.moreRight = false;
    }
    so->numKilled = 0;          /* just paranoia */
+   so->markItemIndex = -1;     /* ditto */
 
    /*
     * Now load data from the first page of the scan.

diff --git a/src/include/access/nbtree.h b/src/include/access/nbtree.h
index e460bbb0d50cf2bf39a50c14b3b512b0510734b9..33d295a71b24b034e26d3117f567e61e9704780d 100644 (file)
--- a/src/include/access/nbtree.h
+++ b/src/include/access/nbtree.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/nbtree.h,v 1.103 2006/08/07 16:57:57 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/nbtree.h,v 1.104 2006/08/24 01:18:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -438,6 +438,15 @@ typedef struct BTScanOpaqueData
    int        *killedItems;    /* currPos.items indexes of killed items */
    int         numKilled;      /* number of currently stored items */
 
+   /*
+    * If the marked position is on the same page as current position,
+    * we don't use markPos, but just keep the marked itemIndex in
+    * markItemIndex (all the rest of currPos is valid for the mark position).
+    * Hence, to determine if there is a mark, first look at markItemIndex,
+    * then at markPos.
+    */
+   int         markItemIndex;  /* itemIndex, or -1 if not valid */
+
    /* keep these last in struct for efficiency */
    BTScanPosData currPos;      /* current position data */
    BTScanPosData markPos;      /* marked position, if any */