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Backport of the fix for BUG#33730 "Full table scan instead selected partitions for query more than 10 partitions"

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from 6.0, made in sergefp@mysql.com-20090205190644-q8632sniogedhtsu
This commit is contained in:
Guilhem Bichot
2009-10-05 22:59:19 +02:00
parent 42e8077834
commit 56312dc7cf
4 changed files with 58 additions and 14 deletions
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32
sql/sql_partition.cc
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@@ -6850,7 +6850,7 @@ int get_part_iter_for_interval_via_mapping(partition_info *part_info,
/* See get_part_iter_for_interval_via_walking for definition of what this is */
#define MAX_RANGE_TO_WALK 10
#define MAX_RANGE_TO_WALK 32
/*
@@ -6886,16 +6886,6 @@ int get_part_iter_for_interval_via_mapping(partition_info *part_info,
Intervals with +inf/-inf, and [NULL, c1] interval can be processed but
that is more tricky and I don't have time to do it right now.
Additionally we have these requirements:
* number of values in the interval must be less then number of
[sub]partitions, and
* Number of values in the interval must be less then MAX_RANGE_TO_WALK.
The rationale behind these requirements is that if they are not met
we're likely to hit most of the partitions and traversing the interval
will only add overhead. So it's better return "all partitions used" in
that case.
RETURN
0 - No matching partitions, iterator not initialized
1 - Some partitions would match, iterator intialized for traversing them
@@ -6989,8 +6979,24 @@ int get_part_iter_for_interval_via_walking(partition_info *part_info,
a += test(flags & NEAR_MIN);
b += test(!(flags & NEAR_MAX));
ulonglong n_values= b - a;
if (n_values > total_parts || n_values > MAX_RANGE_TO_WALK)
/*
Will it pay off to enumerate all values in the [a..b] range and evaluate
the partitioning function for every value? It depends on
1. whether we'll be able to infer that some partitions are not used
2. if time savings from not scanning these partitions will be greater
than time spent in enumeration.
We will assume that the cost of accessing one extra partition is greater
than the cost of evaluating the partitioning function O(#partitions).
This means we should jump at any chance to eliminate a partition, which
gives us this logic:
Do the enumeration if
- the number of values to enumerate is comparable to the number of
partitions, or
- there are not many values to enumerate.
*/
if ((n_values > 2*total_parts) && n_values > MAX_RANGE_TO_WALK)
return -1;
part_iter->field_vals.start= part_iter->field_vals.cur= a;