[db] Add indices on requests db columns

[SeungjinYang]

Separated out from #7602

The database is given an index on created_at. This is because several queries in requests.py have ORDER BY created_at statements, which can be accelerated by an index.

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[SeungjinYang]

Another interesting thing we can do in lieu of adding an index is, given that the request ID is uuid7 now (see #7639), sort by request ID which already has an index.

The pro of this alternate approach is we get to only maintain one index instead of two indices.
The con of this alternate approach is we lose out a bit on precision - python time.time() which is used to generate created_at has microsecond precision on linux, whereas uuid7 timestamp only provides milisecond level precision.

edit: The PR referenced got reverted

[cg505]

should the index also include other fields we filter by? e.g. name, status

[SeungjinYang]

should the index also include other fields we filter by? e.g. name, status

That's a good point. created_at is a no brainer because of ORDER BYs, but we can also index columns used in WHERE clauses.

I took account of what columns we filter most by and found:

• include_request_names / exclude_request_names is specified in RequestTaskFilter a lot, so we can index on name
• cluster_names is filtered on quite a bit as well, so we can index on cluster_name column as well.
• status is also a crowd favorite for filtering as well

So I'm thinking we add index for name and cluster_name, and status column.

[cg505]

cool

[SeungjinYang]

sqlite> PRAGMA index_list(requests);
0|status_idx|0|c|0
1|name_idx|0|c|0
2|cluster_name_idx|0|c|0
3|created_at_idx|0|c|0
4|sqlite_autoindex_requests_1|1|pk|0