spark技术分享WindowsSubstitution Logical Evaluation Rule
WindowsSubstitution is a logical evaluation rule (i.e. Rule[LogicalPlan]) that the logical query plan analyzer uses to resolve (aka substitute) WithWindowDefinition unary logical operators with UnresolvedWindowExpression to their corresponding WindowExpression with resolved WindowSpecDefinition.
WindowsSubstitution is part of Substitution fixed-point batch of rules.
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Note
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It appears that WindowsSubstitution is exclusively used for pure SQL queries because WithWindowDefinition unary logical operator is created exclusively when AstBuilder parses window definitions.
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If a window specification is not found, WindowsSubstitution fails analysis with the following error:
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Window specification [windowName] is not defined in the WINDOW clause. |
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Note
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The analysis failure is unlikely to happen given AstBuilder builds a lookup table of all the named window specifications defined in a SQL text and reports a ParseException when a WindowSpecReference is not available earlier.
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For every WithWindowDefinition, WindowsSubstitution takes the child logical plan and transforms its UnresolvedWindowExpression expressions to be a WindowExpression with a window specification from the WINDOW clause (see WithWindowDefinition Example).
