Known issues

### Things that don't quite work out of the box
- Sorting by RAND()
- You can make this work with a custom script if you so choose
- Sorting by post__in, post_parent__in
- You can probably make this work with a custom script if you so choose
- Sort by meta_value
- Need to think through this one a little more
- Query by week, w, dayofyear, dayofweek
- This is in the works, but depends heavily on how the data is indexed. There needs to be a way to enable/disable this.
- Can't meta compare 'REGEXP', 'NOT REGEXP', 'RLIKE'
- This could probably be done, but I have to think through the implications first.
### Things that don't work at all
- Meta value casting
- There's no equivalent in ES, I just don't think this is something that will be possible in any capacity.
### Noteworthy
- Some tests from core were failing because they were written to assume that two posts with the same date, when ordered by date, would show up in the order in which they were added to the database. However, in ES, they aren't guaranteed to show in that order. tl;dr: unspecified post orders aren't the same between MySQL and ES and can lead to unexpected results.