# Subquery Optimization - the Apply Operator

+ For correlated subqueries, TiDB use Apply operator 
+ `Apply` Operator
  + Takes each row r of the input set R (generally the outer table)
  + Brings it into the expression E(r), where E represents the inner subquery operation,
  + Evaluates the result of E(r). 

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For correlated subqueries, TiDB uses the Apply operator to transform subqueries into relational algebra, a process known as normalization. Following this, the query is de-correlated based on cost information.