Back-patch recent fixes for gistchoose and gistRelocateBuildBuffersOnSplit.

This back-ports commits c8ba697a4bdb934f0c51424c654e8db6133ea255 and
e5db11c5582b469c04a11f217a0f32c827da5dd7, which fix one definite and one
speculative bug in gistchoose, and make the code a lot more intelligible as
well.  In 9.2 only, this also affects the largely-copied-and-pasted logic
in gistRelocateBuildBuffersOnSplit.

The impact of the bugs was that the functions might make poor decisions
as to which index tree branch to push a new entry down into, resulting in
GiST index bloat and poor performance.  The fixes rectify these decisions
for future insertions, but a REINDEX would be needed to clean up any
existing index bloat.

Alexander Korotkov, Robert Haas, Tom Lane

diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 48c44157be07b4d50b8f064fd403f0343e3b4ab3..5ab1e9c7152136759be93ee6f96221ebe98e45cc 100644 (file)
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -372,36 +372,55 @@ gistgetadjusted(Relation r, IndexTuple oldtup, IndexTuple addtup, GISTSTATE *gis
 }
 
 /*
- * find entry with lowest penalty
+ * Search an upper index page for the entry with lowest penalty for insertion
+ * of the new index key contained in "it".
+ *
+ * Returns the index of the page entry to insert into.
  */
 OffsetNumber
 gistchoose(Relation r, Page p, IndexTuple it,  /* it has compressed entry */
           GISTSTATE *giststate)
 {
+   OffsetNumber result;
    OffsetNumber maxoff;
    OffsetNumber i;
-   OffsetNumber which;
-   float       sum_grow,
-               which_grow[INDEX_MAX_KEYS];
+   float       best_penalty[INDEX_MAX_KEYS];
    GISTENTRY   entry,
                identry[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
 
-   maxoff = PageGetMaxOffsetNumber(p);
-   *which_grow = -1.0;
-   which = InvalidOffsetNumber;
-   sum_grow = 1;
+   Assert(!GistPageIsLeaf(p));
+
    gistDeCompressAtt(giststate, r,
                      it, NULL, (OffsetNumber) 0,
                      identry, isnull);
 
+   /* we'll return FirstOffsetNumber if page is empty (shouldn't happen) */
+   result = FirstOffsetNumber;
+
+   /*
+    * The index may have multiple columns, and there's a penalty value for
+    * each column.  The penalty associated with a column that appears earlier
+    * in the index definition is strictly more important than the penalty of
+    * a column that appears later in the index definition.
+    *
+    * best_penalty[j] is the best penalty we have seen so far for column j,
+    * or -1 when we haven't yet examined column j.  Array entries to the
+    * right of the first -1 are undefined.
+    */
+   best_penalty[0] = -1;
+
+   /*
+    * Loop over tuples on page.
+    */
+   maxoff = PageGetMaxOffsetNumber(p);
    Assert(maxoff >= FirstOffsetNumber);
-   Assert(!GistPageIsLeaf(p));
 
-   for (i = FirstOffsetNumber; i <= maxoff && sum_grow; i = OffsetNumberNext(i))
+   for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
    {
-       int         j;
        IndexTuple  itup = (IndexTuple) PageGetItem(p, PageGetItemId(p, i));
+       bool        zero_penalty;
+       int         j;
 
        if (!GistPageIsLeaf(p) && GistTupleIsInvalid(itup))
        {
@@ -411,41 +430,70 @@ gistchoose(Relation r, Page p, IndexTuple it, /* it has compressed entry */
            continue;
        }
 
-       sum_grow = 0;
+       zero_penalty = true;
+
+       /* Loop over index attributes. */
        for (j = 0; j < r->rd_att->natts; j++)
        {
            Datum       datum;
            float       usize;
            bool        IsNull;
 
+           /* Compute penalty for this column. */
            datum = index_getattr(itup, j + 1, giststate->tupdesc, &IsNull);
            gistdentryinit(giststate, j, &entry, datum, r, p, i,
                           FALSE, IsNull);
            usize = gistpenalty(giststate, j, &entry, IsNull,
                                &identry[j], isnull[j]);
+           if (usize > 0)
+               zero_penalty = false;
 
-           if (which_grow[j] < 0 || usize < which_grow[j])
+           if (best_penalty[j] < 0 || usize < best_penalty[j])
+           {
+               /*
+                * New best penalty for column.  Tentatively select this tuple
+                * as the target, and record the best penalty.  Then reset the
+                * next column's penalty to "unknown" (and indirectly, the
+                * same for all the ones to its right).  This will force us to
+                * adopt this tuple's penalty values as the best for all the
+                * remaining columns during subsequent loop iterations.
+                */
+               result = i;
+               best_penalty[j] = usize;
+
+               if (j < r->rd_att->natts - 1)
+                   best_penalty[j + 1] = -1;
+           }
+           else if (best_penalty[j] == usize)
            {
-               which = i;
-               which_grow[j] = usize;
-               if (j < r->rd_att->natts - 1 && i == FirstOffsetNumber)
-                   which_grow[j + 1] = -1;
-               sum_grow += which_grow[j];
+               /*
+                * The current tuple is exactly as good for this column as the
+                * best tuple seen so far.  The next iteration of this loop
+                * will compare the next column.
+                */
            }
-           else if (which_grow[j] == usize)
-               sum_grow += usize;
            else
            {
-               sum_grow = 1;
+               /*
+                * The current tuple is worse for this column than the best
+                * tuple seen so far.  Skip the remaining columns and move on
+                * to the next tuple, if any.
+                */
+               zero_penalty = false;   /* so outer loop won't exit */
                break;
            }
        }
-   }
 
-   if (which == InvalidOffsetNumber)
-       which = FirstOffsetNumber;
+       /*
+        * If we find a tuple with zero penalty for all columns, there's no
+        * need to examine remaining tuples; just break out of the loop and
+        * return it.
+        */
+       if (zero_penalty)
+           break;
+   }
 
-   return which;
+   return result;
 }
 
 /*