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planner: fix bugs related to TIDB_INLJ hint #11253

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merged 9 commits into from
Jul 22, 2019
Merged

planner: fix bugs related to TIDB_INLJ hint #11253

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f268cf1
fix logic of extractTableName and tryToGetIndexJoin
foreyes Jul 15, 2019
c23b6b1
add unit test
foreyes Jul 15, 2019
b3e6506
fix test case
foreyes Jul 15, 2019
277a20e
add comment
foreyes Jul 16, 2019
943444a
Merge branch 'master' into foreyes_dev
foreyes Jul 16, 2019
41c1828
rewrite comments
foreyes Jul 17, 2019
e96d277
rewrite comment
foreyes Jul 18, 2019
74216c7
rewrite comments & clear code
foreyes Jul 22, 2019
a9b60c9
Merge branch 'master' into foreyes_dev
foreyes Jul 22, 2019
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12 changes: 9 additions & 3 deletions planner/core/exhaust_physical_plans.go
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Expand Up @@ -1029,15 +1029,21 @@ func (p *LogicalJoin) tryToGetIndexJoin(prop *property.PhysicalProperty) (indexJ
}

if leftJoins != nil && lhsCardinality < rhsCardinality {
return leftJoins, hasIndexJoinHint
return leftJoins, leftOuter
}

if rightJoins != nil && rhsCardinality < lhsCardinality {
return rightJoins, hasIndexJoinHint
return rightJoins, rightOuter
}

// At this position, at least one of leftJoins and rightJoins is nil
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They can both be not nil? consider the situation that leftJoins != nil && rightJoins != nil && lhsCardinality == rhsCardinality

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This case have been listed in the next line of comments. Do I need to explain more specifically in the comments?
In this case, leftOuter and rightOuter must be both false or both true, and the result of enforced are all correct in these cases.

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I think this comment can be removed, it explains nothing useful for the reviewer to understand the following code logic.

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Okay, It's really confusing.

// Unless (leftOuter == rightOuter && lhsCardinality == rhsCardinality)
canForceLeft := leftJoins != nil && leftOuter
canForceRight := rightJoins != nil && rightOuter

joins := append(leftJoins, rightJoins...)
return joins, hasIndexJoinHint && len(joins) != 0
forced = canForceLeft || canForceRight
return joins, forced
}

return nil, false
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6 changes: 6 additions & 0 deletions planner/core/logical_plan_builder.go
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Expand Up @@ -312,6 +312,12 @@ func (p *LogicalJoin) extractOnCondition(conditions []expression.Expression, der

func extractTableAlias(p LogicalPlan) *model.CIStr {
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if p.Schema().Len() > 0 && p.Schema().Columns[0].TblName.L != "" {
tblName := p.Schema().Columns[0].TblName.L
for _, column := range p.Schema().Columns {
if column.TblName.L != tblName {
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return nil
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What about returns all alias table name here. Then index lookup join hint may be effective in more situation.

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This feature will be added later, we will have some Join Order Hints. By the way, index lookup join doesn't seem to be able to work when there are multiple alias.

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Because there is no available index in this situation.

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tidb(localhost:4000) > drop table if exists t;
Query OK, 0 rows affected (0.02 sec)

tidb(localhost:4000) > create table t(a int, b int, primary key(a));
Query OK, 0 rows affected (0.00 sec)

tidb(localhost:4000) > explain select /*+ TIDB_INLJ(t1) */ t1.b, t2.a from t t1, t t2, t t3 where t1.a = t2.a and t1.a = t3.a;
+----------------------------+----------+------+------------------------------------------------------------+
| id                         | count    | task | operator info                                              |
+----------------------------+----------+------+------------------------------------------------------------+
| Projection_9               | 15625.00 | root | test.t1.b, test.t2.a                                       |
| └─MergeJoin_10             | 15625.00 | root | inner join, left key:test.t3.a, right key:test.t1.a        |
|   ├─TableReader_17         | 10000.00 | root | data:TableScan_16                                          |
|   │ └─TableScan_16         | 10000.00 | cop  | table:t3, range:[-inf,+inf], keep order:true, stats:pseudo |
|   └─MergeJoin_18           | 12500.00 | root | inner join, left key:test.t1.a, right key:test.t2.a        |
|     ├─TableReader_25       | 10000.00 | root | data:TableScan_24                                          |
|     │ └─TableScan_24       | 10000.00 | cop  | table:t1, range:[-inf,+inf], keep order:true, stats:pseudo |
|     └─TableReader_27       | 10000.00 | root | data:TableScan_26                                          |
|       └─TableScan_26       | 10000.00 | cop  | table:t2, range:[-inf,+inf], keep order:true, stats:pseudo |
+----------------------------+----------+------+------------------------------------------------------------+
9 rows in set, 1 warning (0.00 sec)

Actually, in this case, t2.a and t3.a are indices. But we can't choose the index join with TIDB_INLJ hint.

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Because Hint only influence joins involving t1 , it should not enforce parent join to use Index Join. The parent join compares Index Join + Index Join(Hint works) and Merge Join + Merge Join(Hint doesn't work because Merge Join require order, t1 can't be inner table in this situation), and choose the second as the best one.

If change the condition into t1.a = t2.a AND t2.a = t3.a, optimizer will choose Merge Join + Index Join, because t1 is able to be inner table in this situation. Actually, they are the same...Maybe we can do some optimize?

Kenan & me are going to enhance Optimizer Hints.

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Use Hint TIDB_INLJ(t1, t3) may lead to Index Join + Index Join.

}
}
return &(p.Schema().Columns[0].TblName)
}
return nil
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49 changes: 49 additions & 0 deletions planner/core/physical_plan_test.go
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Expand Up @@ -1526,3 +1526,52 @@ func (s *testPlanSuite) TestUnmatchedTableInHint(c *C) {
}
}
}

func (s *testPlanSuite) TestIndexJoinHint(c *C) {
defer testleak.AfterTest(c)()
store, dom, err := newStoreWithBootstrap()
c.Assert(err, IsNil)
defer func() {
dom.Close()
store.Close()
}()
se, err := session.CreateSession4Test(store)
c.Assert(err, IsNil)
_, err = se.Execute(context.Background(), "use test")
c.Assert(err, IsNil)

tests := []struct {
sql string
best string
warning string
}{
{
sql: "select /*+ TIDB_INLJ(t1) */ t1.a, t2.a, t3.a from t t1, t t2, t t3 where t1.a = t2.a and t2.a = t3.a;",
best: "MergeInnerJoin{TableReader(Table(t))->IndexJoin{TableReader(Table(t))->TableReader(Table(t))}(test.t2.a,test.t1.a)}(test.t3.a,test.t2.a)->Projection",
warning: "",
},
{
sql: "select /*+ TIDB_INLJ(t1) */ t1.b, t2.a from t t1, t t2 where t1.b = t2.a;",
best: "LeftHashJoin{TableReader(Table(t))->TableReader(Table(t))}(test.t1.b,test.t2.a)",
warning: "[planner:1815]Optimizer Hint /*+ TIDB_INLJ(t1) */ is inapplicable",
},
}
for i, test := range tests {
comment := Commentf("case:%v sql:%s", i, test)
stmt, err := s.ParseOneStmt(test.sql, "", "")
c.Assert(err, IsNil, comment)

p, err := planner.Optimize(se, stmt, s.is)
c.Assert(err, IsNil)
c.Assert(core.ToString(p), Equals, test.best)

warnings := se.GetSessionVars().StmtCtx.GetWarnings()
if test.warning == "" {
c.Assert(len(warnings), Equals, 0)
} else {
c.Assert(len(warnings), Equals, 1)
c.Assert(warnings[0].Level, Equals, stmtctx.WarnLevelWarning)
c.Assert(warnings[0].Err.Error(), Equals, test.warning)
}
}
}