pgsql support for AST

[staabm]

even if sqlftw does not yet support pgsql we can at least narrow types for pgsql when the statement at hand is compatible with mysql. I think we could run queries thru the sqlftw parser on pgsql but just stop trying to narrow it, if the sql parser throws.

• prio 1: adjust existing tests and make them compatible with pgsql and mysql if possible
• skip tests which are mysql specific
• duplicate mysql specific tests to a pgsql variant

[staabm]

don't let yourself be confused by this "help wanted" label. I just removed this label from the issue tracker, as all issues in the trackers are welcome to be fixed by contributors

[hemberger]

Is #538 a duplicate of this?