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MDEV-4120: UNIQUE indexes should not be considered for loose index scan

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Currently the loose scan code in opt_range.cc considers all indexes as
possible for the access method. Due to inexact statistics it may happen
that a loose scan is selected over a unique index.
  
This is clearly wrong since a "loose scan" over a unique index will read
the same keys as a direct index scan, but the loose scan has more overhead.
  
This task adds a rule to skip unique indexes for loose scan.
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unknown
2013-08-21 10:51:08 +03:00
parent 3c75146052
commit 33c66eb7fb
5 changed files with 59 additions and 18 deletions
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8
sql/opt_range.cc
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@ -12411,8 +12411,12 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree, double read_time)
uchar cur_key_infix[MAX_KEY_LENGTH];
uint cur_used_key_parts;
/* Check (B1) - if current index is covering. */
if (!table->covering_keys.is_set(cur_index))
/*
Check (B1) - if current index is covering. Exclude UNIQUE indexes, because
loose scan may still be chosen for them due to imperfect cost calculations.
*/
if (!table->covering_keys.is_set(cur_index) ||
cur_index_info->flags & HA_NOSAME)
goto next_index;
/*