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toc_title: PREWHERE
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toc_title: PREWHERE
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# PREWHERE Clause {#prewhere}
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# PREWHERE Clause {#prewhere-clause}
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Prewhere is an optimization to apply filtering more efficiently. It is enabled by default even if `PREWHERE` clause is not specified explicitly. It works by automatically moving part of [WHERE](../../../sql-reference/statements/select/where.md) condition to prewhere stage. The role of `PREWHERE` clause is only to control this optimization if you think that you know how to do it better than it happens by default.
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Prewhere is an optimization to apply filtering more efficiently. It is enabled by default even if `PREWHERE` clause is not specified explicitly. It works by automatically moving part of [WHERE](../../../sql-reference/statements/select/where.md) condition to prewhere stage. The role of `PREWHERE` clause is only to control this optimization if you think that you know how to do it better than it happens by default.
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