wiedld commented on code in PR #9685:
URL: https://github.com/apache/arrow-datafusion/pull/9685#discussion_r1531581836
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datafusion/optimizer/src/common_subexpr_eliminate.rs:
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@@ -42,6 +41,15 @@ use datafusion_expr::{col, Expr, ExprSchemable};
/// - DataType of this expression.
type ExprSet = HashMap<Identifier, (Expr, usize, DataType)>;
+/// An ordered map of Identifiers encountered during visitation.
+///
+/// Is created in the ExprIdentifierVisitor, which identifies the common
expressions.
+/// Is consumed in the CommonSubexprRewriter, which performs mutations.
+///
+/// - series_number. increased in fn_up, start from 1.
+/// - Identifier. is empty ("") if expr should not be considered for common
elimation.
+type IdArray = Vec<(usize, Identifier)>;
Review Comment:
This abstraction really helps to understand how the 2 visitors work
together.
The first visitor builds the `IdArray` and `ExprSet`. The first visitor sets
the vector idx used on f_down, whereas the series_number is set on f_up. An
empty string for the expr_id means that this expression is not considered for
rewrite (is skipped) in the second visitor.
e.g. `[(3, expr_id),(2, expr_id),(1, expr_id)]`
The second visitor then performs the mutation, based upon the `IdArray` and
`ExprSet`.
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