babelfish-for-postgresql / babelfish_extensions / 12407215696

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Retain result type after UNION as base type in T-SQL (#3175) (#3268)

Description
-----------
When a base type such as `money` is UNIONed with another integer type,
the result type will be `fixeddecimal` instead of `money`, which is
inconsistent with SQL Server's behaviour. And could also cause some
characters not displaying well.

Analysis
--------
The root cause of the issue is Postgres will consider the base type of
domain type as the result type. In Babelfish, `money` is implemented
as a domain type of `fixeddecimal`. Actually, `money` is a base type
in T-SQL, thus Babelfish should retain `money` type after UNION.

Fix
---
After a final type has determined by the optimizer, check if it's a
base type in T-SQL. If it is, try to use the domain type as result
type if the domain type has higher precision than other types.

Task: BABEL-5242

Signed-off-by: Bo Li <zboli@amazon.com>

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