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MDEV-34720: Poor plan choice for large JOIN with ORDER BY and small LIMIT

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(Variant 2b: call greedy_search() twice, correct handling for limited
 search_depth)

Modify the join optimizer to specifically try to produce join orders that
can short-cut their execution for ORDER BY..LIMIT clause.

The optimization is controlled by @@optimizer_join_limit_pref_ratio.
Default value 0 means don't construct short-cutting join orders.
Other value means construct short-cutting join order, and prefer it only
if it promises speedup of more than #value times.

In Optimizer Trace, look for these names:
* join_limit_shortcut_is_applicable
* join_limit_shortcut_plan_search
* join_limit_shortcut_choice
This commit is contained in:
Sergei Petrunia
2024-08-18 20:10:19 +03:00
parent 819765a47d
commit c8d040938a
9 changed files with 1110 additions and 11 deletions
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sql/sql_class.h
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@ -760,6 +760,7 @@ typedef struct system_variables
ulong net_retry_count;
ulong net_wait_timeout;
ulong net_write_timeout;
ulonglong optimizer_join_limit_pref_ratio;
ulong optimizer_prune_level;
ulong optimizer_search_depth;
ulong optimizer_selectivity_sampling_limit;