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Fix logic bug in gistchoose and gistRelocateBuildBuffersOnSplit.

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Every time the best-tuple-found-so-far changes, we need to reset all
the penalty values in which_grow[] to the penalties for the new best
tuple.  The old code failed to do this, resulting in inferior index
quality.

The original patch from Alexander Korotkov was just two lines; I took
the liberty of fleshing that out by adding a bunch of comments that I
hope will make this logic easier for others to understand than it was
for me.
This commit is contained in:
Robert Haas
2012-08-30 13:05:45 -04:00
parent d1a4db8d25
commit c8ba697a4b
2 changed files with 80 additions and 8 deletions
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43
src/backend/access/gist/gistbuildbuffers.c
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@ -625,8 +625,13 @@ gistRelocateBuildBuffersOnSplit(GISTBuildBuffers *gfbb, GISTSTATE *giststate,
}
/*
* Loop through all index tuples on the buffer on the splitted page,
* moving them to buffers on the new pages.
* Loop through all index tuples on the buffer on the page being split,
* moving them to buffers on the new pages. We try to move each tuple
* the page that will result in the lowest penalty for the leading column
* or, in the case of a tie, the lowest penalty for the earliest column
* that is not tied.
*
* The guts of this loop are very similar to gistchoose().
*/
while (gistPopItupFromNodeBuffer(gfbb, &oldBuf, &itup))
{
@ -637,14 +642,18 @@ gistRelocateBuildBuffersOnSplit(GISTBuildBuffers *gfbb, GISTSTATE *giststate,
IndexTuple newtup;
RelocationBufferInfo *targetBufferInfo;
/*
* Choose which page this tuple should go to.
*/
gistDeCompressAtt(giststate, r,
itup, NULL, (OffsetNumber) 0, entry, isnull);
which = -1;
*which_grow = -1.0f;
/*
* Loop over possible target pages. We'll exit early if we find an index key that
* can accommodate the new key with no penalty on any column. sum_grow is used to
* track this condition. It doesn't need to be exactly accurate, just >0 whenever
* we want the loop to continue and equal to 0 when we want it to terminate.
*/
sum_grow = 1.0f;
for (i = 0; i < splitPagesCount && sum_grow; i++)
@ -653,6 +662,8 @@ gistRelocateBuildBuffersOnSplit(GISTBuildBuffers *gfbb, GISTSTATE *giststate,
RelocationBufferInfo *splitPageInfo = &relocationBuffersInfos[i];
sum_grow = 0.0f;
/* Loop over index attributes. */
for (j = 0; j < r->rd_att->natts; j++)
{
float usize;
@ -664,16 +675,36 @@ gistRelocateBuildBuffersOnSplit(GISTBuildBuffers *gfbb, GISTSTATE *giststate,
if (which_grow[j] < 0 || usize < which_grow[j])
{
/*
* We get here in two cases. First, we may have just discovered that the
* current tuple is the best one we've seen so far; that is, for the first
* column for which the penalty is not equal to the best tuple seen so far,
* this one has a lower penalty than the previously-seen one. But, when
* a new best tuple is found, we must record the best penalty value for
* all the remaining columns. We'll end up here for each remaining index
* column in that case, too.
*/
which = i;
which_grow[j] = usize;
if (j < r->rd_att->natts - 1 && i == 0)
if (j < r->rd_att->natts - 1)
which_grow[j + 1] = -1;
sum_grow += which_grow[j];
}
else if (which_grow[j] == usize)
{
/*
* 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.
*/
sum_grow += usize;
}
else
{
/*
* 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.
*/
sum_grow = 1;
break;
}