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MDEV-37901: Wrong result on QUICK_GROUP_MIN_MAX_SELECT WITH TIES
The "FETCH FIRST n ROWS WITH TIES" was not enforced when the SELECT used "using index for group-by". This was caused by an optimization which removed the ORDER BY clause when the GROUP BY clause prescribed a compatible ordering. Other GROUP BY strategies used workarounds to still handle WITH TIES, see comment to using_with_ties_and_group_min_max() in this patch for details. QUICK_GROUP_MIN_MAX_SELECT didn't have a workaround. Fix this by disabling removal of ORDER BY when QUICK_GROUP_MIN_MAX_SELECT is used.
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‎mysql-test/main/fetch_first.result‎

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@@ -1406,3 +1406,88 @@ a b
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3 bar
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DROP TABLE t1;
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#
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# MDEV-37901: Wrong result with Loose Scan on QUICK_GROUP_MIN_MAX_SELECT WITH TIES
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#
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create table t1 (
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country varchar(100),
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city varchar(100),
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user_score int,
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index (country, city, user_score)
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);
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insert into t1
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select 'China', 'Shenzhen', seq from seq_10_to_100;
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insert into t1
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select 'India', 'New Delhi', seq from seq_10_to_100;
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insert into t1
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select 'Sri Lanka', 'Colombo', seq from seq_10_to_100;
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analyze table t1 persistent for all;
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Table Op Msg_type Msg_text
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test.t1 analyze status Engine-independent statistics collected
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test.t1 analyze status OK
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insert into t1
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select 'Finland', 'Espoo', seq from seq_10_to_200;
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insert into t1
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select 'Greece', 'Chania', seq from seq_10_to_20;
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insert into t1
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select 'Estonia', 'Narva', seq from seq_10_to_20;
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insert into t1
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select 'Russia', 'St Petersburg', seq from seq_10_to_20;
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# Must use "Using index for group-by":
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explain
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select country, city, min(user_score)
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from t1
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where user_score between 9 and 199
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group by country, city
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order by country
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fetch first 5 rows with ties;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 range NULL country 211 NULL 4 Using where; Using index for group-by; Using temporary; Using filesort
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# Must not use "Using index for group-by":
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explain
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select country, city, sum(user_score)
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from t1
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where user_score between 9 and 199
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group by country, concat(city,'AA')
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order by country
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fetch first 5 rows with ties;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 index NULL country 211 NULL 273 Using where; Using index; Using temporary; Using filesort
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select country, city, sum(user_score)
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from t1
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where user_score between 9 and 199
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group by country, concat(city,'AA')
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order by country
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fetch first 5 rows with ties;
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country city sum(user_score)
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China Shenzhen 5005
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Estonia Narva 165
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Finland Espoo 19855
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Greece Chania 165
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India New Delhi 5005
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# both using index and index with group by should produce same result
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select country, city, min(user_score)
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from t1
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where user_score between 9 and 199
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group by country, city
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order by country
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fetch first 5 rows with ties;
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country city min(user_score)
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China Shenzhen 10
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Estonia Narva 10
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Finland Espoo 10
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Greece Chania 10
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India New Delhi 10
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select country, city, min(user_score)
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from t1 use index()
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where user_score between 9 and 199
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group by country, city
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order by country
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fetch first 5 rows with ties;
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country city min(user_score)
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China Shenzhen 10
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Estonia Narva 10
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Finland Espoo 10
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Greece Chania 10
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India New Delhi 10
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drop table t1;

‎mysql-test/main/fetch_first.test‎

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@@ -1082,3 +1082,77 @@ SELECT DISTINCT a, b FROM t1 ORDER BY a FETCH FIRST 3 ROWS WITH TIES;
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# Cleanup
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DROP TABLE t1;
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--echo #
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--echo # MDEV-37901: Wrong result with Loose Scan on QUICK_GROUP_MIN_MAX_SELECT WITH TIES
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--echo #
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--source include/have_sequence.inc
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create table t1 (
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country varchar(100),
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city varchar(100),
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user_score int,
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index (country, city, user_score)
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);
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insert into t1
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select 'China', 'Shenzhen', seq from seq_10_to_100;
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insert into t1
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select 'India', 'New Delhi', seq from seq_10_to_100;
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insert into t1
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select 'Sri Lanka', 'Colombo', seq from seq_10_to_100;
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analyze table t1 persistent for all;
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insert into t1
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select 'Finland', 'Espoo', seq from seq_10_to_200;
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insert into t1
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select 'Greece', 'Chania', seq from seq_10_to_20;
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insert into t1
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select 'Estonia', 'Narva', seq from seq_10_to_20;
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insert into t1
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select 'Russia', 'St Petersburg', seq from seq_10_to_20;
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--echo # Must use "Using index for group-by":
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explain
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select country, city, min(user_score)
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from t1
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where user_score between 9 and 199
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group by country, city
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order by country
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fetch first 5 rows with ties;
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--echo # Must not use "Using index for group-by":
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explain
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select country, city, sum(user_score)
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from t1
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where user_score between 9 and 199
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group by country, concat(city,'AA')
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order by country
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fetch first 5 rows with ties;
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select country, city, sum(user_score)
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from t1
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where user_score between 9 and 199
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group by country, concat(city,'AA')
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order by country
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fetch first 5 rows with ties;
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--echo # both using index and index with group by should produce same result
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select country, city, min(user_score)
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from t1
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where user_score between 9 and 199
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group by country, city
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order by country
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fetch first 5 rows with ties;
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select country, city, min(user_score)
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from t1 use index()
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where user_score between 9 and 199
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group by country, city
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order by country
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fetch first 5 rows with ties;
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drop table t1;

‎sql/opt_range.cc‎

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@@ -15227,6 +15227,46 @@ TRP_GROUP_MIN_MAX::make_quick(PARAM *param, bool retrieve_full_rows,
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}
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/*
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@brief
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Return true if the select is using "Using index for group-by" and also
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has "ORDER BY ... FETCH FIRST n ROWS WITH TIES"
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@detail
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There is a rewrite that removes the ORDER BY (JOIN::order) if the select
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also has a GROUP BY that produces a compatible ordering.
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However "FETCH FIRST ... WITH TIES" needs an ORDER BY clause (in
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JOIN::alloc_order_fields()).
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GROUP BY strategies handle it this way:
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- For strategies using temporary table, JOIN::make_aggr_tables_info() will
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put the ORDER BY clause back.
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- OrderedGroupBy in end_send_group() handles WITH TIES with the GROUP BY
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clause (note that SQL doesn't allow "GROUP BY ... WITH TIES").
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- The remaining strategy is QUICK_GROUP_MIN_MAX_SELECT, for which
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= the grouping strategy in the quick select doesn't handle WITH TIES.
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= end_send() would not handle WITH TIES, because JOIN::order is removed.
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The solution is to NOT remove ORDER BY when QUICK_GROUP_MIN_MAX_SELECT is
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used.
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Unfortunately, the optimizer then will not recognize that it can skip
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sorting and will use filesort, which will prevent short-cutting the
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execution when LIMIT is reached.
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*/
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bool using_with_ties_and_group_min_max(JOIN *join)
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{
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if (join->unit->lim.is_with_ties())
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{
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JOIN_TAB *tab= &join->join_tab[join->const_tables];
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if (tab->select && tab->select->quick &&
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tab->select->quick->get_type() ==
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QUICK_SELECT_I::QS_TYPE_GROUP_MIN_MAX)
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return true;
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}
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return false;
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}
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/*
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Construct new quick select for group queries with min/max.
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‎sql/opt_range.h‎

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Explain_quick_select *get_explain(MEM_ROOT *alloc) override;
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};
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bool using_with_ties_and_group_min_max(JOIN *join);
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class QUICK_SELECT_DESC: public QUICK_RANGE_SELECT
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{

‎sql/sql_select.cc‎

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@@ -3182,9 +3182,14 @@ int JOIN::optimize_stage2()
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(as MariaDB is by default sorting on GROUP BY) or
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if there is no GROUP BY and aggregate functions are used
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(as the result will only contain one row).
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(1) - Do not remove ORDER BY if we have WITH TIES and are using
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QUICK_GROUP_MIN_MAX_SELECT to handle GROUP BY. See the comment
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for using_with_ties_and_group_min_max() for details.
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*/
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if (order && (test_if_subpart(group_list, order) ||
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(!group_list && tmp_table_param.sum_func_count)))
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(!group_list && tmp_table_param.sum_func_count)) &&
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!using_with_ties_and_group_min_max(this)) // (1)
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order=0;
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// Can't use sort on head table if using join buffering
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if (build_tmp_join_prefix_cond(join, tab, &sel->cond))
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return true;
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/*
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We can't call sel->cond->fix_fields,
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as it will break tab->on_expr if it's AND condition

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