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planner: remove constant sort items after substitution (#9333) #9335

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Feb 18, 2019
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planner: remove constant sort items after substitution (#9333) #9335

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5 changes: 5 additions & 0 deletions cmd/explaintest/r/topn_push_down.result
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Original file line number Diff line number Diff line change
Expand Up @@ -183,3 +183,8 @@ Projection_12 0.00 root te.expect_time
│ └─TableScan_31 10.00 cop table:te, keep order:false, stats:pseudo
└─IndexReader_135 10.00 root index:IndexScan_134
└─IndexScan_134 10.00 cop table:p, index:relate_id, range: decided by [tr.id], keep order:false, stats:pseudo
desc select 1 as a from dual order by a limit 1;
id count task operator info
Projection_7 1.00 root 1
└─Limit_8 1.00 root offset:0, count:1
└─TableDual_11 1.00 root rows:1
2 changes: 2 additions & 0 deletions cmd/explaintest/t/topn_push_down.test
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Expand Up @@ -171,3 +171,5 @@ WHERE
te.expect_time BETWEEN '2018-04-23 00:00:00.0' AND '2018-04-23 23:59:59.0'
ORDER BY te.expect_time asc
LIMIT 0, 5;

desc select 1 as a from dual order by a limit 1;
8 changes: 8 additions & 0 deletions planner/core/rule_topn_push_down.go
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Expand Up @@ -96,6 +96,14 @@ func (p *LogicalProjection) pushDownTopN(topN *LogicalTopN) LogicalPlan {
for _, by := range topN.ByItems {
by.Expr = expression.ColumnSubstitute(by.Expr, p.schema, p.Exprs)
}

// remove meaningless constant sort items.
for i := len(topN.ByItems) - 1; i >= 0; i-- {
_, isConst := topN.ByItems[i].Expr.(*expression.Constant)
if isConst {
topN.ByItems = append(topN.ByItems[:i], topN.ByItems[i+1:]...)
}
}
}
p.children[0] = p.children[0].pushDownTopN(topN)
return p
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