Support specifying ASC/DESC in index columns

[miuy56dc]

...by reusing OrderByExpr for columns in Statement::CreateIndex.

This supports SQLite's indexed-column syntax https://www.sqlite.org/syntax/indexed-column.html

MSSQL's (ON <object> ( column [ ASC | DESC ] [ ,...n ] ))
https://docs.microsoft.com/en-us/sql/t-sql/statements/create-index-transact-sql?view=sql-server-ver15

And most of PostgreSQL syntax (except for opclass):
( { column_name | ( expression ) } [ COLLATE collation ] [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
https://www.postgresql.org/docs/12/sql-createindex.html

[coveralls]

Pull Request Test Coverage Report for Build 188045566

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 91.8%

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Totals
Change from base Build 186377189:	0.06%
Covered Lines:	4411
Relevant Lines:	4805

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💛 - Coveralls

[nickolay]

Please refer to the previous discussion in new PRs.

I still don't see why not re-use self.parse_comma_separated(Parser::parse_order_by_expr) instead of introducing another AST type. OrderByExpr should be able to express all of sqlite's options, and Postgres NULLS FIRST as well.

[nickolay]

Thanks! Any reason you prefer indexed_columns to simply columns or is it a left-over from the previous version of the PR?

[miuy56dc]

Because I think column-def and indexed-column are different, so I use indexed-columns to distinguish the difference, but it can be implied to columns because I define it inside of CreateIndex.

[nickolay]

Great. Thanks!