Defer more expressions in vectorized groupBy. #16338
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This patch adds a way for columns to provide GroupByVectorColumnSelectors, which controls how the groupBy engine operates on them. This mechanism is used by ExpressionVirtualColumn to provide an ExpressionDeferredGroupByVectorColumnSelector that uses the inputs of an expression as the grouping key. The actual expression evaluation is deferred until the grouped ResultRow is created. A new context parameter "deferExpressionDimensions" allows users to control when this deferred selector is used. The default is "fixedWidthNonNumeric", which is a behavioral change from the prior behavior. Users can get the prior behavior by setting this to "singleString".
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processing/src/main/java/org/apache/druid/segment/VirtualColumn.java
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LakshSingla
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May 10, 2024
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There's also SqlGroupByBenchmark that benchmarks the code with various distributions and cardinalities. Maybe we should benchmark the code with the string columns and different parameters.
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Are you suggesting adding a new @Benchmark method to SqlGroupByBenchmark that uses a SQL query with expressions?
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This patch adds a way for columns to provide GroupByVectorColumnSelectors, which controls how the groupBy engine operates on them. This mechanism is used by ExpressionVirtualColumn to provide an ExpressionDeferredGroupByVectorColumnSelector that uses the inputs of an expression as the grouping key. The actual expression evaluation is deferred until the grouped ResultRow is created.
A new context parameter
deferExpressionDimensionsallows users to control when this deferred selector is used. The default isfixedWidthNonNumeric, which is a behavioral change from the prior behavior. Users can get the prior behavior by setting this tosingleString.Benchmarks of a few selected queries from
SqlExpressionBenchmarkfollow. Findings:GROUP BY CONCAT(string2, '-', long2), speeds up when the expression is deferred.GROUP BY TIME_FLOOR(TIMESTAMPADD(DAY, -1, __time),GROUP BY long1 * long2,GROUP BY CAST(long1 as BOOLEAN) AND CAST (long2 as BOOLEAN), andGROUP BY long5 IS NULL, long3 IS NOT NULL. All are simple expressions with numeric inputs and outputs.For these reasons, I think
fixedWidthNonNumericis a good default.