allow DateTimeField::Custom with EXTRACT in Postgres

[samuelcolvin]

I'm really not a fan of this dialect_of! stuff, it feels like I'm back writing some kind of odd Python. (I've changed this)

Follows #1191.

This will allow us to match postgres syntax:

postgres@localhost:postgres> select extract(seconds from '2 seconds'::interval);
╒══════════╕
│ extract  │
╞══════════╡
│ 2.000000 │
╘══════════╛
SELECT 1
Time: 0.007s
postgres@localhost:postgres> select extract('seconds' from '2 seconds'::interval);
╒══════════╕
│ extract  │
╞══════════╡
│ 2.000000 │
╘══════════╛
SELECT 1
Time: 0.007s
postgres@localhost:postgres> select extract(minutes from '2 seconds'::interval);
╒═════════╕
│ extract │
╞═════════╡
│ 0       │
╘═════════╛
SELECT 1
Time: 0.007s
postgres@localhost:postgres> select extract(ms from '2 seconds'::interval);
╒══════════╕
│ extract  │
╞══════════╡
│ 2000.000 │
╘══════════╛
SELECT 1
Time: 0.006s
postgres@localhost:postgres> select extract('ms' from '2 seconds'::interval);
╒══════════╕
│ extract  │
╞══════════╡
│ 2000.000 │
╘══════════╛
SELECT 1
Time: 0.006s

I guess all the unit names and abbreviations allowed by INTERVAL which I added in apache/arrow-rs#6211 are allowed, we should be able to use IntervalUnit in datafusion to process the custom value.

[coveralls]

Pull Request Test Coverage Report for Build 10512598473

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 59 of 69 (85.51%) changed or added relevant lines in 6 files are covered.
• 374 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.02%) to 89.156%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/dialect/generic.rs	2	4	50.0%
src/dialect/mod.rs	2	4	50.0%
src/dialect/postgresql.rs	2	4	50.0%
src/dialect/snowflake.rs	2	4	50.0%
tests/sqlparser_common.rs	49	51	96.08%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_clickhouse.rs	7	98.83%
src/ast/ddl.rs	63	86.32%
src/ast/query.rs	92	86.27%
src/parser/mod.rs	212	93.28%

Totals
Change from base Build 10418247467:	0.02%
Covered Lines:	28430
Relevant Lines:	31888

---

💛 - Coveralls

[samuelcolvin]

I think this is good to go.

[iffyio]

LGTM! cc @alamb

[alamb]

Thank you @samuelcolvin and @iffyio