Support table functions with pushdown to connector

[kasiafi]

#1839

This PR introduces partial support for Table Functions (including Polymorphic Table Functions).

Temporarily, only constant scalar arguments are supported (no Table or Descriptor arguments).
The supported execution path for a Table Function is to be pushed down into connector, and replaced with a TableHandle.

Later, Descriptor and Table arguments will be supported, and operator-based execution will be enabled for Table Functions.

[kasiafi]

Sadly, not all jdbc-based connectors want to cooperate. Testing all to see what they need.

[martint]

Ideally, the formatting of each argument should be done by a recursive call to format each element, instead of switching on the specific concrete classes here.

[kasiafi]

Done for all except Expression arguments. I order to be processed recursively, they would require wrapping in another AST node, which I considered too much overhead.

[martint]

It'd be better to model these as sub-rules so that the construction occurs naturally by the recursive tree traversal instead of switching on whether qualifiedName is present or not

[kasiafi]

done

[martint]

Why is the argument a list of names?

[kasiafi]

Refactored so that the list is implementation detail in table function resolution.

[martint]

There should be explicit validation to make sure the arguments provided to the function are not duplicated. This will catch that condition, but with an unclear error message.

[martint]

applyFunction should return a TableHandle and a list of ColumnHandle for that TH corresponding to the columns coming out of the original table function. This optimizer should not have to derive that from the table metadata, which may not actually exist for a "virtual" table.

[martint]

Why does this hold a name?

[kasiafi]

Refactored. Now the TableFunctionHandle holds the catalog name.

[kasiafi]

@martint Applied comments

[Praveen2112]

As a follow up - can we extract it to a common util method ?

[martint]

Great job. Let's merge everything in except for the functions added to the jdbc and elasticsearch connectors. We need to come up with a good name for them and make sure they have reasonable semantics -- I haven't had a chance to review them yet.

[wendigo]

Woah, nice job @kasiafi!

[mosabua]

I am going to assume this needs to release notes entry or docs at this stage. Going forward can you maybe use the PR template to indicate that @kasiafi .. thanks

[kasiafi]

I am going to assume this needs to release notes entry or docs at this stage.

@mosabua this does need a release notes entry and docs. I'll add the proposed release notes entry in the relnotes issue. I'm currently working on the documentation -- it's likely we'll have to wait for it until the next release.