Support setting visibility on ALTER COLUMN.
#4809
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MySQL 8.0 introduced the [ability to hide and unhide columns]. Columns
can be hidden in a `CREATE TABLE` statement (currently unimplemented in
`sqlglot`) and can be subsequently changed with an `ALTER TABLE` syntax:
`ALTER COLUMN x SET {VISIBLE|INVISIBLE}`.
This commit adds support for column hiding through the `ALTER TABLE`
syntax. However, it does so in a MySQL-specific way. Other RDBMSs have
similar support for column hiding, but implement it slightly
differently:
* Oracle uses `MODIFY x INVISIBLE`
* DB2 uses `ALTER x t IMPLICITLY HIDDEN`
* SQLServer uses `ALTER COLUMN x ADD HIDDEN`
As there is no other dialect in `sqlglot` that implements invisibility,
and because the parser currently maintains dialect-specific
implementations, I wasn't sure if the amount of effort to refactor the
parser was worthwhile for this change.
[ability to hide and unhide columns]: https://dev.mysql.com/doc/refman/8.4/en/alter-table.html
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MySQL 8.0 introduced the ability to hide and unhide columns. Columns can be hidden in a
CREATE TABLEstatement (currently unimplemented insqlglot) and can be subsequently changed with anALTER TABLEsyntax:ALTER COLUMN x SET {VISIBLE|INVISIBLE}.This commit adds support for column hiding through the
ALTER TABLEsyntax. However, it does so in a MySQL-specific way. Other RDBMSs have similar support for column hiding, but implement it slightly differently:MODIFY x INVISIBLEALTER x t IMPLICITLY HIDDENALTER x ADD HIDDENAs there is no other dialect in
sqlglotthat implements invisibility, and because the parser currently maintains other dialect-specific implementations, I wasn't sure if the amount of effort to refactor the parser to be dialect-agnostic was worthwhile for this specific change (I couldn't find a similar pattern). If you disagree, I would be willing to attempt the refactor, though, I would appreciate some suggestions on how it could be done, as it was not immediately apparent.