ENH: Store in SQL using double precision

Closes #9009 . @jorisvandenbossche -- do you think this should test/use single precision ?  We could test for float32 perhaps? My own feeling is that this is the safer default.
Can you maybe leave this out? 
In any case, this is to fix #9083, so it should also have a test associated with it. And I also already have a patch for this (a bit cleaner I think, it's what I mentioned in #9083), but not yet pushed it as a PR. Sorry for not doing that directly, I will do that now.