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Commit9fd8843

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Fix mergejoin cost estimation so that we consider the statistical ranges of
the two join variables at both ends: not only trailing rows that need not bescanned because there cannot be a match on the other side, but initial rowsthat will be scanned without possibly having a match. This allows a morerealistic estimate of startup cost to be made, per recent pgsql-performancediscussion. In passing, fix a couple of bugs that had crept intomergejoinscansel: it was not quite up to speed for the task of estimatingdescending-order scans, which is a new requirement in 8.3.
1 parent8821612 commit9fd8843

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4 files changed

+345
-148
lines changed

4 files changed

+345
-148
lines changed

‎src/backend/optimizer/path/costsize.c

Lines changed: 78 additions & 31 deletions
Original file line numberDiff line numberDiff line change
@@ -54,7 +54,7 @@
5454
* Portions Copyright (c) 1994, Regents of the University of California
5555
*
5656
* IDENTIFICATION
57-
* $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.189 2007/11/15 22:25:15 momjian Exp $
57+
* $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.190 2007/12/08 21:05:11 tgl Exp $
5858
*
5959
*-------------------------------------------------------------------------
6060
*/
@@ -1372,12 +1372,16 @@ cost_mergejoin(MergePath *path, PlannerInfo *root)
13721372
doubleouter_path_rows=PATH_ROWS(outer_path);
13731373
doubleinner_path_rows=PATH_ROWS(inner_path);
13741374
doubleouter_rows,
1375-
inner_rows;
1375+
inner_rows,
1376+
outer_skip_rows,
1377+
inner_skip_rows;
13761378
doublemergejointuples,
13771379
rescannedtuples;
13781380
doublerescanratio;
1379-
Selectivityouterscansel,
1380-
innerscansel;
1381+
Selectivityouterstartsel,
1382+
outerendsel,
1383+
innerstartsel,
1384+
innerendsel;
13811385
Selectivityjoininfactor;
13821386
Pathsort_path;/* dummy for result of cost_sort */
13831387

@@ -1444,10 +1448,12 @@ cost_mergejoin(MergePath *path, PlannerInfo *root)
14441448
* A merge join will stop as soon as it exhausts either input stream
14451449
* (unless it's an outer join, in which case the outer side has to be
14461450
* scanned all the way anyway). Estimate fraction of the left and right
1447-
* inputs that will actually need to be scanned. We use only the first
1448-
* (most significant) merge clause for this purpose. Since
1449-
* mergejoinscansel() is a fairly expensive computation, we cache the
1450-
* results in the merge clause RestrictInfo.
1451+
* inputs that will actually need to be scanned. Likewise, we can
1452+
* estimate the number of rows that will be skipped before the first
1453+
* join pair is found, which should be factored into startup cost.
1454+
* We use only the first (most significant) merge clause for this purpose.
1455+
* Since mergejoinscansel() is a fairly expensive computation, we cache
1456+
* the results in the merge clause RestrictInfo.
14511457
*/
14521458
if (mergeclauses&&path->jpath.jointype!=JOIN_FULL)
14531459
{
@@ -1478,37 +1484,61 @@ cost_mergejoin(MergePath *path, PlannerInfo *root)
14781484
outer_path->parent->relids))
14791485
{
14801486
/* left side of clause is outer */
1481-
outerscansel=cache->leftscansel;
1482-
innerscansel=cache->rightscansel;
1487+
outerstartsel=cache->leftstartsel;
1488+
outerendsel=cache->leftendsel;
1489+
innerstartsel=cache->rightstartsel;
1490+
innerendsel=cache->rightendsel;
14831491
}
14841492
else
14851493
{
14861494
/* left side of clause is inner */
1487-
outerscansel=cache->rightscansel;
1488-
innerscansel=cache->leftscansel;
1495+
outerstartsel=cache->rightstartsel;
1496+
outerendsel=cache->rightendsel;
1497+
innerstartsel=cache->leftstartsel;
1498+
innerendsel=cache->leftendsel;
14891499
}
14901500
if (path->jpath.jointype==JOIN_LEFT)
1491-
outerscansel=1.0;
1501+
{
1502+
outerstartsel=0.0;
1503+
outerendsel=1.0;
1504+
}
14921505
elseif (path->jpath.jointype==JOIN_RIGHT)
1493-
innerscansel=1.0;
1506+
{
1507+
innerstartsel=0.0;
1508+
innerendsel=1.0;
1509+
}
14941510
}
14951511
else
14961512
{
14971513
/* cope with clauseless or full mergejoin */
1498-
outerscansel=innerscansel=1.0;
1514+
outerstartsel=innerstartsel=0.0;
1515+
outerendsel=innerendsel=1.0;
14991516
}
15001517

1501-
/* convert selectivity to row count; must scan at least one row */
1502-
outer_rows=clamp_row_est(outer_path_rows*outerscansel);
1503-
inner_rows=clamp_row_est(inner_path_rows*innerscansel);
1518+
/*
1519+
* Convert selectivities to row counts. We force outer_rows and
1520+
* inner_rows to be at least 1, but the skip_rows estimates can be zero.
1521+
*/
1522+
outer_skip_rows=rint(outer_path_rows*outerstartsel);
1523+
inner_skip_rows=rint(inner_path_rows*innerstartsel);
1524+
outer_rows=clamp_row_est(outer_path_rows*outerendsel);
1525+
inner_rows=clamp_row_est(inner_path_rows*innerendsel);
1526+
1527+
Assert(outer_skip_rows <=outer_rows);
1528+
Assert(inner_skip_rows <=inner_rows);
15041529

15051530
/*
15061531
* Readjust scan selectivities to account for above rounding. This is
15071532
* normally an insignificant effect, but when there are only a few rows in
15081533
* the inputs, failing to do this makes for a large percentage error.
15091534
*/
1510-
outerscansel=outer_rows /outer_path_rows;
1511-
innerscansel=inner_rows /inner_path_rows;
1535+
outerstartsel=outer_skip_rows /outer_path_rows;
1536+
innerstartsel=inner_skip_rows /inner_path_rows;
1537+
outerendsel=outer_rows /outer_path_rows;
1538+
innerendsel=inner_rows /inner_path_rows;
1539+
1540+
Assert(outerstartsel <=outerendsel);
1541+
Assert(innerstartsel <=innerendsel);
15121542

15131543
/* cost of source data */
15141544

@@ -1522,14 +1552,18 @@ cost_mergejoin(MergePath *path, PlannerInfo *root)
15221552
outer_path->parent->width,
15231553
-1.0);
15241554
startup_cost+=sort_path.startup_cost;
1555+
startup_cost+= (sort_path.total_cost-sort_path.startup_cost)
1556+
*outerstartsel;
15251557
run_cost+= (sort_path.total_cost-sort_path.startup_cost)
1526-
*outerscansel;
1558+
*(outerendsel-outerstartsel);
15271559
}
15281560
else
15291561
{
15301562
startup_cost+=outer_path->startup_cost;
1563+
startup_cost+= (outer_path->total_cost-outer_path->startup_cost)
1564+
*outerstartsel;
15311565
run_cost+= (outer_path->total_cost-outer_path->startup_cost)
1532-
*outerscansel;
1566+
*(outerendsel-outerstartsel);
15331567
}
15341568

15351569
if (innersortkeys)/* do we need to sort inner? */
@@ -1542,14 +1576,18 @@ cost_mergejoin(MergePath *path, PlannerInfo *root)
15421576
inner_path->parent->width,
15431577
-1.0);
15441578
startup_cost+=sort_path.startup_cost;
1579+
startup_cost+= (sort_path.total_cost-sort_path.startup_cost)
1580+
*innerstartsel*rescanratio;
15451581
run_cost+= (sort_path.total_cost-sort_path.startup_cost)
1546-
*innerscansel*rescanratio;
1582+
*(innerendsel-innerstartsel)*rescanratio;
15471583
}
15481584
else
15491585
{
15501586
startup_cost+=inner_path->startup_cost;
1587+
startup_cost+= (inner_path->total_cost-inner_path->startup_cost)
1588+
*innerstartsel*rescanratio;
15511589
run_cost+= (inner_path->total_cost-inner_path->startup_cost)
1552-
*innerscansel*rescanratio;
1590+
*(innerendsel-innerstartsel)*rescanratio;
15531591
}
15541592

15551593
/* CPU costs */
@@ -1571,8 +1609,11 @@ cost_mergejoin(MergePath *path, PlannerInfo *root)
15711609
* joininfactor.
15721610
*/
15731611
startup_cost+=merge_qual_cost.startup;
1612+
startup_cost+=merge_qual_cost.per_tuple*
1613+
(outer_skip_rows+inner_skip_rows*rescanratio);
15741614
run_cost+=merge_qual_cost.per_tuple*
1575-
(outer_rows+inner_rows*rescanratio);
1615+
((outer_rows-outer_skip_rows)+
1616+
(inner_rows-inner_skip_rows)*rescanratio);
15761617

15771618
/*
15781619
* For each tuple that gets through the mergejoin proper, we charge
@@ -1597,8 +1638,10 @@ cached_scansel(PlannerInfo *root, RestrictInfo *rinfo, PathKey *pathkey)
15971638
{
15981639
MergeScanSelCache*cache;
15991640
ListCell*lc;
1600-
Selectivityleftscansel,
1601-
rightscansel;
1641+
Selectivityleftstartsel,
1642+
leftendsel,
1643+
rightstartsel,
1644+
rightendsel;
16021645
MemoryContextoldcontext;
16031646

16041647
/* Do we have this result already? */
@@ -1617,8 +1660,10 @@ cached_scansel(PlannerInfo *root, RestrictInfo *rinfo, PathKey *pathkey)
16171660
pathkey->pk_opfamily,
16181661
pathkey->pk_strategy,
16191662
pathkey->pk_nulls_first,
1620-
&leftscansel,
1621-
&rightscansel);
1663+
&leftstartsel,
1664+
&leftendsel,
1665+
&rightstartsel,
1666+
&rightendsel);
16221667

16231668
/* Cache the result in suitably long-lived workspace */
16241669
oldcontext=MemoryContextSwitchTo(root->planner_cxt);
@@ -1627,8 +1672,10 @@ cached_scansel(PlannerInfo *root, RestrictInfo *rinfo, PathKey *pathkey)
16271672
cache->opfamily=pathkey->pk_opfamily;
16281673
cache->strategy=pathkey->pk_strategy;
16291674
cache->nulls_first=pathkey->pk_nulls_first;
1630-
cache->leftscansel=leftscansel;
1631-
cache->rightscansel=rightscansel;
1675+
cache->leftstartsel=leftstartsel;
1676+
cache->leftendsel=leftendsel;
1677+
cache->rightstartsel=rightstartsel;
1678+
cache->rightendsel=rightendsel;
16321679

16331680
rinfo->scansel_cache=lappend(rinfo->scansel_cache,cache);
16341681

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