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Dmitriy 2020-10-24 13:17:48 +03:00
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docs/en/sql-reference/statements/select/prewhere.md
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toc_title: PREWHERE
---
# PREWHERE Clause {#prewhere}
# PREWHERE Clause {#prewhere-clause}
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.