feat(rust,python,cli): support negative indexing and expressions for LEFT, RIGHT and SUBSTR SQL string funcs

[alexander-beedie]

Description

Builds on top of @reswqa's great work expressifying string slice (see #13747).

• Extends SQL LEFT, RIGHT, and SUBSTR SQL string functions with expression support.
• Adds support for negative length to both the LEFT and RIGHT functions.
• Adds supports for negative start and negative length for SUBSTR.
• Also fixes ROUND error checking for negative 'n_decimals' as a drive-by.

Notes on negative start/length

Getting this correct was a bit headache-inducing, as SQL has a different idea of what negative values mean here and it's 1-indexed.

For example, withSUBSTR:

• Starting at -10 doesn't mean "start 10 chars back from the end of the string", as you would likely expect; instead it really does mean "start 10 chars back from the front". So, SUBSTR(colt,-10,13) and SUBSTR(colx,1,2) both return the same result (due to offsets and 1-indexing), which would be the first two letters of the string 🤔

With the LEFT and RIGHT functions you have the opportunity to set negative length:

• LEFT("s",-n) means "all but the last n chars"
• RIGHT("s",-n) means "all but first n chars"

Testing

This probably took more time than the actual implementation, due to the notable differences between what you'd typically expect (as a programmer) and what the SQL standard actually defines. I ended up with a set of unit tests that I could also run directly on a dockerised PostgreSQL instance to validate the behaviour, covering all major parameter characteristics.

• (Screenshot: having fun confirming RIGHT equivalence) 😅

References

PostgreSQL string function docs:
https://www.postgresql.org/docs/9.1/functions-string.html#FUNCTIONS-STRING-OTHER

[ritchie46]

Good thing we have CSE!

[alexander-beedie]

Definitely! Mapping back to SQL behaviour was awkward enough without having to worry about eating a performance hit too 😅