As maintainer of the SQL Server driver for Drupal I have gathered several issues over these last years regarding Drupal Core using MySQL specific stuff, including but not limited to:
Function CONCAT_WS
Function GROUP_CONCAT
Function GREATEST
Use of MySQL specific data types in SCHEMA definitions
This makes it difficult for Drupal to be database agnostic.
I was able to "workaround" some of these issues in the DB driver (as long as you don't use db_query directly) but some of these issues are impossible to workaround.
Every issue by itself would not be major, but all of them as a whole make running Drupal on other database backends more difficult (I guess....).
There is even one issue where Drupal is using specific MySQL data types to get case sensitivity to work instead of using binary => TRUE. It is not properly using the DTBNG abstraction layer.
#2013028: GREATEST function does not behave equally in all database engines →
#2009238: CONCAT_WS is not compatible with all database drivers →
#1598924: Query with duplicate placeholders throws PDO Exception →
#2375407: Views should not use constant values or expressions in aggregates →
#2490976: Locale caching algorithm is broken on Non MySQL/PostgreSQL databases →
Closed: outdated
11.0 🔥
database system
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
There are 2 open related issues here and one if for the Drupal 7 Views contrib module. By convention, a meta can be closed when there is one remaining issue. Also, there has been quite a bit of work on the database API and moving each database driver to their own module.
So, I am closing that at outdated.