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Commit0ea5eee

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author
Richard Guo
committed
Avoid creating duplicate ordered append paths
In generate_orderedappend_paths(), the function does not handle thecase where the paths in total_subpaths and fractional_subpaths areidentical. This situation is not uncommon, and as a result, it maygenerate two exactly identical ordered append paths.Fix by checking whether total_subpaths and fractional_subpaths containthe same paths, and skipping creation of the ordered append path forthe fractional case when they are identical.Given the lack of field complaints about this, I'm a bit hesitant toback-patch, but let's clean it up in HEAD.Author: Richard Guo <guofenglinux@gmail.com>Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>Discussion:https://postgr.es/m/CAMbWs4-OYsgA75tGGiBARt87G0y_z_GBTSLrzudcJxAzndYkYw@mail.gmail.com
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‎src/backend/optimizer/path/allpaths.c‎

Lines changed: 12 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1877,6 +1877,7 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
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List*total_subpaths=NIL;
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List*fractional_subpaths=NIL;
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boolstartup_neq_total= false;
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boolfraction_neq_total= false;
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boolmatch_partition_order;
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boolmatch_partition_order_desc;
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intend_index;
@@ -2005,15 +2006,21 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
20052006
* XXX We might consider partially sorted paths too (with an
20062007
* incremental sort on top). But we'd have to build all the
20072008
* incremental paths, do the costing etc.
2009+
*
2010+
* Also, notice whether we actually have different paths for
2011+
* the "fractional" and "total" cases. This helps avoid
2012+
* generating two identical ordered append paths.
20082013
*/
2009-
if (!cheapest_fractional)
2014+
if (cheapest_fractional==NULL)
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cheapest_fractional=cheapest_total;
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elseif (cheapest_fractional!=cheapest_total)
2017+
fraction_neq_total= true;
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}
20122019

20132020
/*
20142021
* Notice whether we actually have different paths for the
2015-
* "cheapest" and "total" cases; frequently there will be no point
2016-
*in two create_merge_append_path() calls.
2022+
* "cheapest" and "total" cases. This helps avoid generating two
2023+
*identical ordered append paths.
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*/
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if (cheapest_startup!=cheapest_total)
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startup_neq_total= true;
@@ -2084,7 +2091,7 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
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false,
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-1));
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2087-
if (fractional_subpaths)
2094+
if (fractional_subpaths&&fraction_neq_total)
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add_path(rel, (Path*)create_append_path(root,
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rel,
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fractional_subpaths,
@@ -2110,7 +2117,7 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
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pathkeys,
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NULL));
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2113-
if (fractional_subpaths)
2120+
if (fractional_subpaths&&fraction_neq_total)
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add_path(rel, (Path*)create_merge_append_path(root,
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rel,
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fractional_subpaths,

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