plan,analyzer: Implement a TopRowsNode. #635
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For a query like: `SELECT * FROM table ORDER BY id LIMIT 100 OFFSET 100` we do not need to materialize the entire sorted result. We can use a heap to compute a streaming TopN and output the results when the child iterator is EOF. * Add a simple implementation of TopN based on container/heap and sql/expression.Sorter. * Add a TopRowsNode and topRowIter. * Add a simple analyzer rule to replace a Sort node with a TopRows node when it sits directly underneath a Limit() or directly underneath a Limit(Offset()). Some care is required to keep `sql_calc_found_rows` operating as expected.
zachmu
approved these changes
Nov 18, 2021
sql/analyzer/topn.go
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| package analyzer | ||
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| import ( | ||
| //"fmt" |
sql/plan/sort.go
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| return nil, sql.ErrInvalidChildrenNumber.New(n, len(exprs), len(n.SortFields)) | ||
| } | ||
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| var fields = make([]sql.SortField, len(n.SortFields)) |
There was a problem hiding this comment.
You can use SortFields.FromExpressions for this
sql/plan/sort.go
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| // Expressions implements the Expressioner interface. | ||
| func (n *TopN) Expressions() []sql.Expression { | ||
| var exprs = make([]sql.Expression, len(n.SortFields)) |
There was a problem hiding this comment.
You can use SortFields.ToExpressions for this
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For a query like:
SELECT * FROM table ORDER BY id LIMIT 100 OFFSET 100we do not need to materialize the entire sorted result. We can use a heap to
compute a streaming TopN and output the results when the child iterator is EOF.
Add a simple implementation of TopN based on container/heap and
sql/expression.Sorter.
Add a TopRowsNode and topRowIter.
Add a simple analyzer rule to replace a Sort node with a TopRows node when it
sits directly underneath a Limit() or directly underneath a Limit(Offset()).
Some care is required to keep
sql_calc_found_rowsoperating as expected.