OR filters not supported

[txntxn]

Do you know how I can get Postgres OR working with Multicorn?

I get this message:
WARNING: unsupported expression for extractClauseFrom

Over email @rdunklau asked for a proposal for implementing these quals that is backwards compatible. My proposal to maintain backwards compatibility is as follows:

• Create a new op called "OR_EXPR" or "AST_EXPR".
• This op has a value list, and is_list_operator is true.
• The value of the boolean in the tuple is ANY, but it doesn't matter.
• The value of "field" is either None if that is allowed, or we can select the name of an existing column. Either way, the "field" is unused.
• Since people add code for each op individually, their existing code for skipping unhandled ops will just skip this qual because they will see a new op: "AST_EXPR".
• The value list is a list of Quals.
• The qual performs an OR over all the subordinate Quals.

This approach is nice because existing code can be reused and extended to add the OR_EXPR, and you can recurse.

Over email @rdunklau replied:

"it may be better to offer a separate qual API entirely.

Depending on an attribute defined on the foreign-data-wrapper (QUAL_API = 2)
for example, the qual representation would either be as-is, or be a binary
tree more closely reflecting the internal structure of PostgreSQL expression
trees.

This would allow both to coexist peacefully, while keeping the default of
having everything be simple enough."

[txntxn]

This appears related to #30