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MDEV-17761: Odd optimizer choice with ORDER BY LIMIT and condition selectivity

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Make the "ORDER BY ... LIMIT n" optimizer take into account condition
selectivity data from EITS (not just from potential range accesses).
This commit is contained in:
Sergei Petrunia
2019-01-23 15:49:49 +03:00
parent 36a2a185fe
commit b7a784ae25
5 changed files with 145 additions and 0 deletions
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22
sql/sql_select.cc
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@ -26884,7 +26884,11 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
uint tablenr= (uint)(tab - join->join_tab);
read_time= join->best_positions[tablenr].read_time;
for (uint i= tablenr+1; i < join->table_count; i++)
{
fanout*= join->best_positions[i].records_read; // fanout is always >= 1
// But selectivity is =< 1 :
fanout*= join->best_positions[i].cond_selectivity;
}
}
else
read_time= table->file->scan_time();
@ -27022,6 +27026,23 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
*/
select_limit= (ha_rows) (select_limit < fanout ?
1 : select_limit/fanout);
/*
refkey_rows_estimate is E(#rows) produced by the table access
strategy that was picked without regard to ORDER BY ... LIMIT.
It will be used as the source of selectivity data.
Use table->cond_selectivity as a better estimate which includes
condition selectivity too.
*/
{
// we use MIN(...), because "Using LooseScan" queries have
// cond_selectivity=1 while refkey_rows_estimate has a better
// estimate.
refkey_rows_estimate= MY_MIN(refkey_rows_estimate,
table_records * table->cond_selectivity);
}
/*
We assume that each of the tested indexes is not correlated
with ref_key. Thus, to select first N records we have to scan
@ -27032,6 +27053,7 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
N/(refkey_rows_estimate/table_records) > table_records
<=> N > refkey_rows_estimate.
*/
if (select_limit > refkey_rows_estimate)
select_limit= table_records;
else