I went to write some docs for datetime functions that look like:
```
SELECT YEAR(CAST('2018-01-19T10:23:27Z' AS TIMESTAMP)) as year;
year
2018
```
because I figured they'd be pretty easy to read because they didn't
require any knowledge of a data set. But it turns out that constant
folding doesn't work properly for date time functions because they don't
actually apply the extraction.
Original commit: elastic/x-pack-elasticsearch@aa9c66b2c7