Repair old performance bug in tuplesort.c/logtape.c. In the case where

we are doing the final merge pass on-the-fly, and not writing the data
back onto a 'tape', the number of free blocks in the tape set will become
large, leading to a lot of time wasted in ltsReleaseBlock().  There is
really no need to track the free blocks anymore in this state, so add a
simple shutoff switch.  Per report from Stefan Kaltenbrunner.

diff --git a/src/backend/utils/sort/logtape.c b/src/backend/utils/sort/logtape.c
index 4f280509cadeec0fbbe0c29e41745139b4c10fb3..6dc203063c96a5e8385369b25fb9d759c57b5664 100644 (file)
--- a/src/backend/utils/sort/logtape.c
+++ b/src/backend/utils/sort/logtape.c
@@ -64,7 +64,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/utils/sort/logtape.c,v 1.19 2006/03/05 15:58:49 momjian Exp $
+ *   $PostgreSQL: pgsql/src/backend/utils/sort/logtape.c,v 1.20 2006/03/07 19:06:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -146,7 +146,12 @@ struct LogicalTapeSet
     * When there are no such blocks, we extend the underlying file.  Note
     * that the block numbers in freeBlocks are always in *decreasing* order,
     * so that removing the last entry gives us the lowest free block.
+    *
+    * If forgetFreeSpace is true then any freed blocks are simply forgotten
+    * rather than being remembered in freeBlocks[].  See notes for
+    * LogicalTapeSetForgetFreeSpace().
     */
+   bool        forgetFreeSpace;    /* are we remembering free blocks? */
    long       *freeBlocks;     /* resizable array */
    int         nFreeBlocks;    /* # of currently free blocks */
    int         freeBlocksLen;  /* current allocated length of freeBlocks[] */
@@ -247,6 +252,12 @@ ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
    int         ndx;
    long       *ptr;
 
+   /*
+    * Do nothing if we're no longer interested in remembering free space.
+    */
+   if (lts->forgetFreeSpace)
+       return;
+
    /*
     * Enlarge freeBlocks array if full.
     */
@@ -491,6 +502,7 @@ LogicalTapeSetCreate(int ntapes)
                                    (ntapes - 1) * sizeof(LogicalTape));
    lts->pfile = BufFileCreateTemp(false);
    lts->nFileBlocks = 0L;
+   lts->forgetFreeSpace = false;
    lts->freeBlocksLen = 32;    /* reasonable initial guess */
    lts->freeBlocks = (long *) palloc(lts->freeBlocksLen * sizeof(long));
    lts->nFreeBlocks = 0;
@@ -546,6 +558,21 @@ LogicalTapeSetClose(LogicalTapeSet *lts)
    pfree(lts);
 }
 
+/*
+ * Mark a logical tape set as not needing management of free space anymore.
+ *
+ * This should be called if the caller does not intend to write any more data
+ * into the tape set, but is reading from un-frozen tapes.  Since no more
+ * writes are planned, remembering free blocks is no longer useful.  Setting
+ * this flag lets us avoid wasting time and space in ltsReleaseBlock(), which
+ * is not designed to handle large numbers of free blocks.
+ */
+void
+LogicalTapeSetForgetFreeSpace(LogicalTapeSet *lts)
+{
+   lts->forgetFreeSpace = true;
+}
+
 /*
  * Dump the dirty buffer of a logical tape.
  */

diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
index a9d5d79e033b2a0676a0c5b2592951776aafe0c9..af0109fda653f5a2e31d4536a62451fc06744635 100644 (file)
--- a/src/backend/utils/sort/tuplesort.c
+++ b/src/backend/utils/sort/tuplesort.c
@@ -91,7 +91,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/utils/sort/tuplesort.c,v 1.62 2006/03/05 15:58:49 momjian Exp $
+ *   $PostgreSQL: pgsql/src/backend/utils/sort/tuplesort.c,v 1.63 2006/03/07 19:06:50 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1384,7 +1384,8 @@ mergeruns(Tuplesortstate *state)
    /*
     * If we produced only one initial run (quite likely if the total data
     * volume is between 1X and 2X workMem), we can just use that tape as the
-    * finished output, rather than doing a useless merge.
+    * finished output, rather than doing a useless merge.  (This obvious
+    * optimization is not in Knuth's algorithm.)
     */
    if (state->currentRun == 1)
    {
@@ -1401,33 +1402,51 @@ mergeruns(Tuplesortstate *state)
 
    for (;;)
    {
-       /* Step D5: merge runs onto tape[T] until tape[P] is empty */
-       while (state->tp_runs[state->tapeRange - 1] ||
-              state->tp_dummy[state->tapeRange - 1])
+       /*
+        * At this point we know that tape[T] is empty.  If there's just one
+        * (real or dummy) run left on each input tape, then only one merge
+        * pass remains.  If we don't have to produce a materialized sorted
+        * tape, we can stop at this point and do the final merge on-the-fly.
+        */
+       if (!state->randomAccess)
        {
-           bool        allDummy = true;
            bool        allOneRun = true;
 
+           Assert(state->tp_runs[state->tapeRange] == 0);
            for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
            {
-               if (state->tp_dummy[tapenum] == 0)
-                   allDummy = false;
                if (state->tp_runs[tapenum] + state->tp_dummy[tapenum] != 1)
+               {
                    allOneRun = false;
+                   break;
+               }
            }
-
-           /*
-            * If we don't have to produce a materialized sorted tape, quit as
-            * soon as we're down to one real/dummy run per tape.
-            */
-           if (!state->randomAccess && allOneRun)
+           if (allOneRun)
            {
-               Assert(!allDummy);
+               /* Tell logtape.c we won't be writing anymore */
+               LogicalTapeSetForgetFreeSpace(state->tapeset);
                /* Initialize for the final merge pass */
                beginmerge(state);
                state->status = TSS_FINALMERGE;
                return;
            }
+       }
+
+       /* Step D5: merge runs onto tape[T] until tape[P] is empty */
+       while (state->tp_runs[state->tapeRange - 1] ||
+              state->tp_dummy[state->tapeRange - 1])
+       {
+           bool        allDummy = true;
+
+           for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
+           {
+               if (state->tp_dummy[tapenum] == 0)
+               {
+                   allDummy = false;
+                   break;
+               }
+           }
+
            if (allDummy)
            {
                state->tp_dummy[state->tapeRange]++;
@@ -1437,6 +1456,7 @@ mergeruns(Tuplesortstate *state)
            else
                mergeonerun(state);
        }
+
        /* Step D6: decrease level */
        if (--state->Level == 0)
            break;

diff --git a/src/include/utils/logtape.h b/src/include/utils/logtape.h
index 3791f03e9e3bc620c6a56b4ad73c5e37775fed22..7cd3db3777479d606a0fff07fc328b755da8bc7d 100644 (file)
--- a/src/include/utils/logtape.h
+++ b/src/include/utils/logtape.h
@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/logtape.h,v 1.14 2006/03/05 15:59:07 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/logtape.h,v 1.15 2006/03/07 19:06:50 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,6 +26,7 @@ typedef struct LogicalTapeSet LogicalTapeSet;
 
 extern LogicalTapeSet *LogicalTapeSetCreate(int ntapes);
 extern void LogicalTapeSetClose(LogicalTapeSet *lts);
+extern void LogicalTapeSetForgetFreeSpace(LogicalTapeSet *lts);
 extern size_t LogicalTapeRead(LogicalTapeSet *lts, int tapenum,
                void *ptr, size_t size);
 extern void LogicalTapeWrite(LogicalTapeSet *lts, int tapenum,