Add docs for update statements to JDBC connectors

[colebow]

Description

RN was slightly misleading and connector docs need to mention update support. Knock those both out.

Release notes

(x) This is not user-visible or is docs only, and no release notes are required.

[hashhar]

Not all UPDATE queries would work. Same limitation as DELETE applies.

Only UPDATE queries that can actually be "pushed down" are supported hence the release notes mentioned pushdown.

[mosabua]

Not all UPDATE queries would work. Same limitation as DELETE applies.

Only UPDATE queries that can actually be "pushed down" are supported hence the release notes mentioned pushdown.

How can we explain that to users... how would you know if a update query can be pushed down or not. We might have to follow up with more details for each connector that explain that but we cant talk about pushdown unless we explain how a user can figure that out from writing a SQL query. I also hope that any error messages are meaningful so that users can tell why some update query works and another one does not.

[mosabua]

Any insights @hashhar ? Looks like the PR from @vlad-lyutenko #16445 has more info. Can @vlad-lyutenko please send a PR to update the docs more .. potentially working with @colebow or @jhlodin .. I can help with review and merge.

[vlad-lyutenko]

Maybe I can describe more detailed about limitation:
we support only constant updates, like : UPDATE nation SET col1 = 1 WHERE col3 = 1;
it means that we can not use any expressions in SET section like UPDATE nation SET col1 = col2 + 2 WHERE col3 = 1;

Also we cannot use AND/OR in predicates like UPDATE nation SET col1 = 1 WHERE col3 = 1 AND col2 = 3;

And most tricky limitation - UPDATE ALL, it means -
if we have table with 3 columns - col1 col2 col3
We cannot execute query which update all 3 columns simultaneously, like this:
UPDATE nation SET col1 = 1, col2=2, col3=3 WHERE col3 = 1;
But we can update 2 or 1 of them like:
UPDATE nation SET col1 = 1, col2=2 WHERE col3 = 1;