When TiDB reads data from TiKV, TiDB tries to push down some expressions (including calculations of functions or operators) to be processed to TiKV. This reduces the amount of transferred data and offloads processing from a single TiDB node. This document introduces the expressions that TiDB already supports pushing down and how to prohibit specific expressions from being pushed down using blocklist.
Tiflash also supports pushdown for the functions and operators listed on this page.
If unexpected behavior occurs in the calculation process when pushing down the supported expressions or specific data types (only the
ENUM type and the
BIT type), you can restore the application quickly by prohibiting the pushdown of the corresponding functions, operators, or data types. Specifically, you can prohibit the functions, operators, or data types from being pushed down by adding them to the blocklist
mysql.expr_pushdown_blacklist. For details, refer to Add to the blocklist.