Support deferrable unique constraint for PostgreSQL

[yuku]

This PR will make psqldef handle DDL like:

alter table dummy add constraint dummy_uniq unique (column_a, column_b) deferrable initially deferred;

PostgreSQL: Documentation: 14: ALTER TABLE

[yuku]

@k0kubun In 52f4809 I tried to fix the sqlparser to accept the expected DDL, but when I run the tests I added, I get syntax errors. I spent several hours trying to debug it, but I still haven't been able to, partly because I'm new to goyacc.

Can you see what I'm missing?

By the way, if you think it is much easier for you to write another patch by yourself from scratch rather than reviewing my code, please feel free to close this PR 🙏

cd cmd/psqldef && PGHOST=127.0.0.1 PGSSLMODE=disable go test -v -run TestPsqldefAddConstraintUnique
=== RUN   TestPsqldefAddConstraintUnique
    psqldef_test.go:668: failed to execute './psqldef -Upostgres psqldef_test --file schema.sql' (error: 'exit status 1'): `found syntax error when parsing DDL "alter table dummy add constraint dummy_uniq unique (column_a, column_b) not deferrable initially immediate": syntax error at position 87 near 'deferrable'
        `
    psqldef_test.go:668: expected '-- Nothing is modified --
        ' but got 'found syntax error when parsing DDL "alter table dummy add constraint dummy_uniq unique (column_a, column_b) not deferrable initially immediate": syntax error at position 87 near 'deferrable'
        '
...

[k0kubun]

I tried to fix the sqlparser to accept the expected DDL, but when I run the tests I added, I get syntax errors. I spent several hours trying to debug it, but I still haven't been able to, partly because I'm new to goyacc.

You declared new tokens on the parser side, but you didn't let the lexer generate the tokens. So those keywords were recognized as if they were normal identifiers instead of keywords. Fixing it is as simple as 16aa629.

You also need to touch schema/ast.go and schema/parser.go to complete the generator implementation. However, it's just specific to the current codebase that's a bit messed up. So it's not your fault. I plan to change the architecture of how schema package works, but the work has not been completed yet. Anyway it's fixed in 53ef6c4. I needed to delete a test case because:

2022/01/21 21:09:40 pq: constraint declared INITIALLY DEFERRED must be DEFERRABLE

[k0kubun]

Thanks!

[k0kubun]

Released as v0.11.24.