Function tidb_query_aggr::util::rewrite_exp_for_sum_avg

pub fn rewrite_exp_for_sum_avg(
    schema: &[FieldType], 
    exp: &mut RpnExpression
) -> Result<()>

Rewrites the expression to insert necessary cast functions for SUM and AVG aggregate functions.

See typeInfer4Sum and typeInfer4Avg in TiDB.

TODO: This logic should be performed by TiDB.