Details
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Bug
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Status: Closed (View Workflow)
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Major
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Resolution: Fixed
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10.1.11
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None
Description
The optimizer can't reorder tables for OUTER JOINs, but when a later WHERE condition on a column of an outer table requires a non-NULL value it can convert the OUTER join into an INNER one, and then it can also make use of re-ordering.
The same works when the OUTER JOIN is inside a view definition, and a WHERE condition on a column of the outer table is applied on the view ... but apparently only if the view definition doesn't have a WHERE clause.
How to reproduce:
DROP TABLE IF EXISTS t1;
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CREATE TABLE t1(i1 int primary key, v1 int, key(v1)) engine=myisam;
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INSERT INTO t1 VALUES (1, 1);
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INSERT INTO t1 VALUES (2, 2);
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INSERT INTO t1 VALUES (3, 3);
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INSERT INTO t1 VALUES (4, 4);
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INSERT INTO t1 VALUES (5, 3);
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INSERT INTO t1 VALUES (6, 6);
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INSERT INTO t1 VALUES (7, 7);
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INSERT INTO t1 VALUES (8, 8);
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INSERT INTO t1 VALUES (9, 9);
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DROP TABLE IF EXISTS t2;
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CREATE TABLE t2(i2 int primary key, v2 int, key(v2)) engine=myisam;
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INSERT INTO t2 VALUES (1, 1);
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INSERT INTO t2 VALUES (2, 2);
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INSERT INTO t2 VALUES (3, 3);
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INSERT INTO t2 VALUES (4, 4);
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INSERT INTO t2 VALUES (5, 3);
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INSERT INTO t2 VALUES (6, 6);
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INSERT INTO t2 VALUES (7, 7);
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INSERT INTO t2 VALUES (8, 8);
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INSERT INTO t2 VALUES (9, 9);
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DROP TABLE IF EXISTS t3;
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CREATE TABLE t3(i3 int primary key, v3 int, key(v3)) engine=myisam;
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INSERT INTO t3 VALUES (2, 2);
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INSERT INTO t3 VALUES (4, 4);
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INSERT INTO t3 VALUES (6, 6);
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INSERT INTO t3 VALUES (8, 8);
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# view definition without WHERE
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DROP VIEW IF EXISTS v1;
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CREATE ALGORITHM=MERGE VIEW v1 AS
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SELECT t1.i1 as i1, t1.v1 as v1,
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t2.i2 as i2, t2.v2 as v2,
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t3.i3 as i3, t3.v3 as v3
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FROM t1
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JOIN t2 on t1.i1 = t2.i2
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LEFT JOIN t3 on t2.i2 = t3.i3
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;
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# view definition with WHERE
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DROP VIEW IF EXISTS v2;
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CREATE ALGORITHM=MERGE VIEW v2 AS
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SELECT t1.i1 as i1, t1.v1 as v1,
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t2.i2 as i2, t2.v2 as v2,
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t3.i3 as i3, t3.v3 as v3
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FROM t1 JOIN t2 on t1.i1 = t2.i2
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LEFT JOIN t3 on t2.i2 = t3.i3
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WHERE t1.i1 = t2.i2
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AND 1 = 1
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;
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# query plan of plain query
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EXPLAIN EXTENDED SELECT t1.i1 as i1, t1.v1 as v1,
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t2.i2 as i2, t2.v2 as v2,
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t3.i3 as i3, t3.v3 as v3
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FROM t1
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JOIN t2 on t1.i1 = t2.i2
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LEFT JOIN t3 on t2.i2 = t3.i3
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WHERE 1 = 1
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AND t3.v3 = 4
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;
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# query plan for view without WHERE
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EXPLAIN EXTENDED SELECT * FROM v1 WHERE v3 = 4;
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# query plan for view with WHERE
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EXPLAIN EXTENDED SELECT * FROM v2 WHERE v3 = 4;
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The plans for the plain query, and for the VIEW without a WHERE condition inside the definition, look like this:
Plain query:
id select_type table type possible_keys key key_len ref rows filtered Extra
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1 SIMPLE t3 ref PRIMARY,v3 v3 5 const 1 100.00
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1 SIMPLE t1 eq_ref PRIMARY PRIMARY 4 test.t3.i3 1 100.00
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1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t3.i3 1 100.00
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View without WHERE:
id select_type table type possible_keys key key_len ref rows filtered Extra
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1 SIMPLE t3 ref PRIMARY,v3 v3 5 const 1 100.00
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1 SIMPLE t1 eq_ref PRIMARY PRIMARY 4 test.t3.i3 1 100.00
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1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t3.i3 1 100.00
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So these two are identical.
For the VIEW with the extra WHERE 1=1 in its definition the plan looks like this though:
id select_type table type possible_keys key key_len ref rows filtered Extra
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1 SIMPLE t1 ALL PRIMARY NULL NULL NULL 9 100.00
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1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.i1 1 100.00
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1 SIMPLE t3 eq_ref PRIMARY,v3 PRIMARY 4 test.t1.i1 1 100.00 Using where
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So here table reordering didn't happen, and a "Using where" full scan of the VIEW results has to be done ....
Attachments
1.
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Testing for MDEV-10006 |
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Closed | Elena Stepanova |
Activity
This patch fixes the issue and passes the test suite:
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
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index e83f63d..d9849ce 100644
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--- a/sql/sql_select.cc
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+++ b/sql/sql_select.cc
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@@ -14418,6 +14418,9 @@ static void update_const_equal_items(THD *thd, COND *cond, JOIN_TAB *tab,
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if (table->outer_join && !table->embedding && table->table)
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table->table->maybe_null= FALSE;
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table->outer_join= 0;
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+ //psergey:
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+ if (!(straight_join || table->straight))
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+ table->dep_tables= table->embedding? table->embedding->dep_tables: 0;
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if (table->on_expr)
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{
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/* Add ON expression to the WHERE or upper-level ON condition. */
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elenst, I need
- a test of 10.1 vs 10.1 with the above patch
- we need a grammar with nested LEFT/INNER joins, with various ON expressions
- ON expression must be simple predicates over null-able as well as NOT NULL columns
- The primary risk (thing to check for) is different query results. I don't expect crashes.
I ran comparison tests on the patched version against 10.1 without the patch and transformation tests with ExecuteAsView, haven't got any mismatches.
The testing task MDEV-10031 will remain open until the patch passes the review, in case there are changes that require retesting.
No changes were requested during the code review. The patch will be pushed as is.
Debugging simplify_joins:
Enter simplify_joins(join_list= join->join_list= {w2}, conds= "t3.v3 = 4")table= w2,table->on_expr= ((t1.i1 = t2.i2) and (1 = 1))table->nested_join= ...Enter simplify_joins(join_list= w2->nested_join={(nest_last_join)},conds="((t1.i1 = t2.i2) and (1 = 1))"{table= (nest_last_join) // denote as $nest1table->on_expr=NULL,Enter simplify_joins(join_list= $nest1->nested_join={t3, (nest_last_join)},conds="((t1.i1 = t2.i2) and (1 = 1))"{table= t3table->on_expr= (t2.i2 = t3.i3)table->outer_join=1// we assign:table->dep_tables |= table_on_expr_used_tables// and it becomes:table->dep_tables=6// 6 = 4 (for t3 itself) + 2 (for t2)// interesting:table->dep_tables&= ~table->embedding->nested_join->used_tables;// here the right part has used_tables=4// although embedding's nested join contains t3 and a join nest// which includes t1 and t2. I suppose, that used_tables value is incomplete// ATM.// Anyhow, after this, we havetable->dep_tables=2// that is, "t3 must follow t2" which looks correct (outer join is not// converted to inner, yet).table= (nest_last_join); // denote as $nest2table->on_expr= 0;table->nested_join= {t1, t2}Enter simplify_joins(join_list= $nest2->nested_join={t2, t1},conds="((t1.i1 = t2.i2) and (1 = 1))"{table= t2table->on_expr= "(t1.i1 = t2.i2)"table->outer_join= 0// inject table->on_expr into conds.table= t1retval= "((t1.i1 = t2.i2) and (1 = 1) and (t1.i1 = t2.i2))"} // Leave simplify_joins(join_list=$nest2...)// The list {t3, (nest_last_join)} is flattened into// {t3, t2, t1}retval= "((t1.i1 = t2.i2) and (1 = 1) and (t1.i1 = t2.i2))"} // Leave simplify_joins(join_list=$nest1...)// the list {nest_last_join(t3, t2, t1)} is flattened into// {t3,t2,t1}retval= "((t1.i1 = t2.i2) and (1 = 1) and (t1.i1 = t2.i2))"} // Leave simplify_joins// Now, the second call to simplify joins, with 'conds' as parameter:Enter simplify_joins(join_list= w2->nested_join={t3, t2, t1},conds="(t3.v3 = 4)"{table= t3used_tables=4, not_null_tables=4// we set: table->outer_join= 0;// however, table->dep_tables=2 remains uncleanedsame content here: https://gist.github.com/spetrunia/d531a08dc748fb01c694710885ac2bc7
Summary: