I'm wondering if there's one case that could be considered here that isn't:

SELECT * FROM MyTable  WHERE NOT flag; -- inverted var

I could be wrong, but I don't think this is either a BooleanTest or a Var, but would be a BoolExpr, so that would be a distinct thing to take on.

A very small nit but it looks like there's a mix of tab & space indentation in the newly added code, if you don't mind standardizing it to match the source file that would be amazing.

I think I have fixed it.

It would be great if you could add to one of the test suites all of the supported cases -- multicorn_regression_test.sql would be ideal as there's a small Test quals section there that could be expanded with the new boolean operations to ensure that they work in all environments and are maintained in the future.

I would love to add tests, but I have not yet succeeded in running the existing test suite.

I would love to add tests, but I have not yet succeeded in running the existing test suite.

I run a little unusual dev environment (running NixOS) so I can't advise a whole lot on this -- in NixOS, or if Nix is installed, it's documented in the README (https://github.com/pgsql-io/multicorn2?tab=readme-ov-file#integration-tests). But outside of that environment... theoretically it should just require make easycheck?

If you have troubles, we could merge this in and I can add a test... but I took a quick shot at it and ran into a concern you might want to review first. I added these test statements near the end of test-3.9/sql/multicorn_regression_test.sql...

...
SELECT * from testmulticorn where test3 = 12::money;
SELECT * from testmulticorn where test1 = '12 €';

-- Test boolean operation pushdown
ALTER FOREIGN TABLE testmulticorn alter test1 type bool;
select * from testmulticorn where test1;
select * from testmulticorn where test1 = TRUE;
select * from testmulticorn where test1 = FALSE;
select * from testmulticorn where test1 IS TRUE;
select * from testmulticorn where test1 IS FALSE;
select * from testmulticorn where test1 IS UNKNOWN;
select * from testmulticorn where test1 IS NOT UNKNOWN;

DROP USER MAPPING FOR current_user SERVER multicorn_srv;
DROP EXTENSION multicorn cascade;
...

Resulted in a warning and no quals being pushed down, in the test1 = FALSE case:

select * from testmulticorn where test1 = FALSE;
WARNING:  unsupported expression for extractClauseFrom
DETAIL:  {BOOLEXPR :boolop not :args ({VAR :varno 1 :varattno 1 :vartype 16 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnosyn 1 :varattnosyn 1 :location 76}) :location -1}
WARNING:  unsupported expression for extractClauseFrom
DETAIL:  {BOOLEXPR :boolop not :args ({VAR :varno 1 :varattno 1 :vartype 16 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnosyn 1 :varattnosyn 1 :location 76}) :location -1}

I'm wondering if there's one case that could be considered here that isn't:

SELECT * FROM MyTable  WHERE NOT flag; -- inverted var

I could be wrong, but I don't think this is either a BooleanTest or a Var, but would be a BoolExpr, so that would be a distinct thing to take on.

Done.

Resulted in a warning and no quals being pushed down, in the test1 = FALSE case:

Using EXPLAIN, it appears the test1 = FALSE is normalized to NOT test1, which is covered by my recent update.

run a little unusual dev environment (running NixOS) so I can't advise a whole lot on this -- in NixOS, or if Nix is installed, it's documented in the README (https://github.com/pgsql-io/multicorn2?tab=readme-ov-file#integration-tests). But outside of that environment... theoretically it should just require make easycheck?

I wasn't ready to cut a PR for this, but I have playing with adding a dev.container.
https://github.com/pgsql-io/multicorn2/compare/main...bovlb:multicorn2:dev-container?expand=1
In this environment, I can neither run the nix commands suggested in the readme, nor make easycheck (which I already found).

@bovlb Sorry for the inconvenience on testing -- I'll try to write up some useful dev testing instructions in the future. In the meantime I popped in my tests after your NOT column addition and merged. Thanks for the contribution! 👍