SQL: Add CAST and CONVERT to SHOW FUNCTIONS

[matriv]

Fixes: #34939

[elasticmachine]

Pinging @elastic/es-search-aggs

[matriv]

retest this please

[matriv]

@bpintea I've chosen to show CAST and CONVERT with type: DATA_TYPE_CONVERSION instead of SCALAR as most DBs list CAST in a different section than scalar functions. What do you think? Is it ok for ODBC, or should I list them as SCALAR ?

[costin]

I would stick with SCALAR since essentially that's what they are doing. DATA_TYPE_CONVERSION is a long description (with underscores) and I'm not sure what are the expected grouping names.

[astefan]

LGTM

[bpintea]

Is it ok for ODBC, or should I list them as SCALAR ?

AFA ODBC is concerned, the way we list the function is orthogonal to it; i.e. there's no API which maps directly to this catalog call a.t.m.

AFAI can tell, ODBC lists it however as a scalar function, under the type SQL_CONVERT_FUNCTIONS, where the supported conversions (or "functions") are are to be inquired with types provided in the following paragraph.

A while ago we have decided not to obtain this information from ES/SQL dynamically, but to rather hard coded it in the driver. But even if we were to change that decision, in the listing we now have, the type column would not suffice to cover the API call where it could be relevant (SQLGetInfo()), since the supported conversions are not part of that listing.
This is why we have no direct mapping on this catalog call and why the way we do the listing is atm not relevant to the driver.

[matriv]

@bpintea Thanks a lot for looking into this! Changing the type to SCALAR.

[costin]

LGTM

[matriv]

Backported to 6.x with 71704fa

[matriv]

Backported to 6.5 with 7e995e0