feat(optimizer): simplify_equality

[barakalon]

We have expressions like this:

DATE_ADD(
  DATE_ADD(
    DATE_TRUNC(
      'week', 
      DATE_SUB(
        x, 
        1, 
        DAY
      )
    ), 
    1, 
    DAY
  ), 
  1, 
  YEAR
) < CAST('2021-01-08' AS DATE);

This optimization, plus the previous operations, gives:

x < CAST('2020-01-07' AS DATE);

[tobymao]

what would happen if it's

x - interval 1 day = cast(y as date)

[barakalon]

Nothing. The right side must be a date literal.

[tobymao]

are these copies needed?

[barakalon]

ah shit sorry i hit merge.
I'm not sure.
In general, I copy when creating a new expression using child nodes from an existing tree. When do you?

[tobymao]

in the optimizer, i only copy when i have to, because it's mutation heavy and expensive,

in generator, where the contract is idempotency and no mutations, i copy

[barakalon]

#2282

[georgesittas]

Agree on copy being a bit redundant here: It seems like we're dropping the expression arg in this return statement anyway & a and b are not associated with r's AST, so this completely replaces the AST corresponding to the input expression.