We’re in the process of moving our codebase from Postgres to MySQL (using PlanetScale) and we’re noticing a number of our aggregate queries are now trying to calculate using the Decimal<> type, instead of integers.

For example, the following query treats the column types differently depending on which adapter we’re using.

SomeTable
|> select([t], sum(t.int_col_one) - sum(i.int_col_two))
|> Repo.one()

Using Postgres this will subtract two integers.

Using MySQL (using MyXQL) it will attempt to subtract from a decimal.

Error thrown from the query

bad argument in arithmetic expression: Decimal.new("7401") - 7401

:erlang.-(Decimal.new("7401"), 7401)

We have also observed sum and avg aggregates also return decimal types now (all columns are integers) where as previously we would get an integer back when using Postgres.

I haven’t found much info on this so any pointers or solutions would be appreciated.