ES|QL: add MV_APPEND function

[luigidellaquila]

Adding MV_APPEND(value1, value2) function, that appends two values creating a single multi-value. If one or both the inputs are multi-values, the result is the concatenation of all the values, eg.

MV_APPEND([a, b], [c, d]) -> [a, b, c, d]

~I think for this specific case it makes sense to consider null values as empty arrays, so that~

MV_APPEND(value, null) -> value

~It is pretty uncommon for ESQL (all the other functions, apart from COALESCE, short-circuit to null when one of the values is null), so let's discuss this behavior.~

[EDIT] considering the feedback from Andrei, I changed this logic and made it consistent with the other functions: now if one of the parameters is null, the function returns null

[github-actions[bot]]

Documentation preview:

• ✨ Changed pages

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[luigidellaquila]

@elasticmachine update branch

[nik9000]

I think for this specific case it makes sense to consider null values as empty arrays, so that

That's fine - our null is basically an empty list. I'd do it.

[nik9000]

It is pretty uncommon for ESQL (all the other functions, apart from COALESCE, short-circuit to null when one of the values is null), so let's discuss this behavior.

There's a small number that don't - maybe 5 out of the 100.

[luigidellaquila]

@elasticmachine update branch

[luigidellaquila]

@elasticmachine update branch