alamb opened a new issue #1693:
URL: https://github.com/apache/arrow-datafusion/issues/1693
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
In certain situations , IOx is likely going to make predicates that look
like the following
```sql
CASE
WHEN col IS NULL THEN ''
ELSE col
END
```
that basically map `null` to the empty string
When applying them to certain specialized chunks (or row groups) we will
know there are no NULLs in the `col` or all NULLs and thus we will end up
rewriting it to something like
```sql
CASE
WHEN true THEN ''
ELSE col
END
```
Also in general, when applying other simplifications / constant folding I
can imagine other situations where CASE can be folded such as
```sql
CASE
WHEN extract(day) from now() = 0 THEN 'Monday'
WHEN extract(day) from now() = 1 THEN 'Tuesday'
WHEN extract(day) from now() = 2 THEN 'Wednesday'
WHEN extract(day) from now() = 3 THEN 'Monday'
...
ELSE 'other day'
END
```
So I think it is worth adding into DataFusion generally
**Describe the solution you'd like**
I would like to add cases to the rewrite rules here:
https://github.com/apache/arrow-datafusion/blob/03075d5f4b3fdfd8f82144fcd409418832a4bf69/datafusion/src/optimizer/simplify_expressions.rs#L440-L454
Some rules I can think of are:
1. When there is a literal `true` in the `cases` preceded by 0 or 1 literal
falses or nulls, use that.
2. When all the cases are false, --> the otherwise
**Additional context**
See https://github.com/influxdata/influxdb_iox/pull/3557 for more details
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