feat(polars): add limited support for table dot sql

[cpcloud]

Adds limited support for .sql to the Polars backend. Closes #8525.

[lostmygithubaccount]

this at least works for the simple example I want to use in docs:

[ins] In [1]: import ibis

[ins] In [2]: ibis.options.interactive = True

[ins] In [3]: ibis.set_backend("polars")

[ins] In [4]: t = ibis.examples.penguins.fetch()

[ins] In [5]: t
Out[5]:
┏━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━┓
┃ species ┃ island    ┃ bill_length_mm ┃ bill_depth_mm ┃ flipper_length_mm ┃ body_mass_g ┃ sex    ┃ year  ┃
┡━━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━╇━━━━━━━┩
│ string  │ string    │ float64        │ float64       │ int64             │ int64       │ string │ int64 │
├─────────┼───────────┼────────────────┼───────────────┼───────────────────┼─────────────┼────────┼───────┤
│ Adelie  │ Torgersen │           39.1 │          18.7 │               181 │        3750 │ male   │  2007 │
│ Adelie  │ Torgersen │           39.5 │          17.4 │               186 │        3800 │ female │  2007 │
│ Adelie  │ Torgersen │           40.3 │          18.0 │               195 │        3250 │ female │  2007 │
│ Adelie  │ Torgersen │           NULL │          NULL │              NULL │        NULL │ NULL   │  2007 │
│ Adelie  │ Torgersen │           36.7 │          19.3 │               193 │        3450 │ female │  2007 │
│ Adelie  │ Torgersen │           39.3 │          20.6 │               190 │        3650 │ male   │  2007 │
│ Adelie  │ Torgersen │           38.9 │          17.8 │               181 │        3625 │ female │  2007 │
│ Adelie  │ Torgersen │           39.2 │          19.6 │               195 │        4675 │ male   │  2007 │
│ Adelie  │ Torgersen │           34.1 │          18.1 │               193 │        3475 │ NULL   │  2007 │
│ Adelie  │ Torgersen │           42.0 │          20.2 │               190 │        4250 │ NULL   │  2007 │
│ …       │ …         │              … │             … │                 … │           … │ …      │     … │
└─────────┴───────────┴────────────────┴───────────────┴───────────────────┴─────────────┴────────┴───────┘

[ins] In [6]: t.sql("SELECT species, island, count(*) AS count FROM penguins GROUP BY species, island")
Out[6]:
┏━━━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━┓
┃ species   ┃ island    ┃ count  ┃
┡━━━━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━━┩
│ string    │ string    │ uint32 │
├───────────┼───────────┼────────┤
│ Chinstrap │ Dream     │     68 │
│ Adelie    │ Torgersen │     52 │
│ Gentoo    │ Biscoe    │    124 │
│ Adelie    │ Biscoe    │     44 │
│ Adelie    │ Dream     │     56 │
└───────────┴───────────┴────────┘

[cpcloud]

@lostmygithubaccount That only works because the examples fetch method creates a table/view named penguins. If you call .alias(something that isn't "penguins") you won't be able to refer to that new name in subsequent SQL strings.

[cpcloud]

I guess for the docs example it's fine. It might be good to call out the lack of SQL support in .sql for Polars in the docs somewhere.

[lostmygithubaccount]

yeah it's fine for the docs...if users run into issues we can point to the Polars issue(s) on SQL support

[cpcloud]

Ugh, this fails because Polars doesn't respect/allow/handle quoted identifiers.

[cpcloud]

CTEs are implemented but a table with name "t" somehow doesn't equal a table with name t.

[lostmygithubaccount]

might want to have a proper engineer review the code but looks fine to me :)

[gforsyth]

Looks good overall, question about dependencies (our favorite!)

[gforsyth]

I have a few thoughts, but they can be done in a follow-up:

1. I think we should have some kind of try except import check, so we can raise a helpful error message since a user using the polars backend might get thrown by having an "unrelated" import error for psycopg2.
2. Because psycopg2 can be a pain to install (for pip users, at least), does this have to make use of the postgres compiler? Would duckdb work? Because duckdb would always be installed anyway. And that would remove the need for point 1 above, now that I'm thinking about it.

[cpcloud]

The psycopg2 import should never be hit when compiling using dialect="postgres".

Given that we're not installing the postgres extra in the polars CI job, I think we can be reasonably sure we don't have an implicit dependency on it for this functionality.

With that, is there anything to follow up with?

[gforsyth]

You've got an XPASS in the exasol test suite, but other than that