Fix changing a column on PostgreSQL and SQL Server

[JonasPardon]

Following up on this question on #258: the ->change() shim generates invalid SQL on PostgreSQL and SQL Server on wip/1.3. SQLite is fine: it keeps every attribute, apart from ->default(null), which #254 fixes.

I ran the same migrations on PostgreSQL 16.15 against Winter 1.2 (Laravel 9 + DBAL), current wip/1.3, and this branch, and on SQL Server 2022 against wip/1.3 and this branch:

->change() without restating the attribute	Winter 1.2	wip/1.3	this PR
PostgreSQL: any change that doesn't restate nullable()	works	syntax error: alter column "a" not null	works
PostgreSQL: column with a default ('foo', 0.00)	kept	syntax error: alter column "a" default …	kept
PostgreSQL: O'Brien default	can't be created (Laravel 9 doesn't escape the ')	syntax error	kept
PostgreSQL: useCurrent() timestamp, made nullable	fails (DBAL)	syntax error	works, keeps CURRENT_TIMESTAMP
PostgreSQL: comment	kept	syntax error	kept
PostgreSQL: C collation	dropped	syntax error	kept
SQL Server: column with a default	not tested	Incorrect syntax near the keyword 'default'	default added back
SQL Server: Latin1_General_BIN collation	not tested	dropped	kept
SQL Server: two columns changed in one blueprint	not tested	6 statements, each column altered twice	2 statements per column

Causes:

• The old column is built from getColumns() without the change flag, so Laravel's PostgreSQL modifiers compile it as a new column (null, default 'foo') instead of drop not null and set default. On SQL Server, modifyDefault() adds an inline default, which alter column doesn't accept.
• The reported default was passed through as a string, but both servers report defaults as SQL expressions ('foo'::character varying, ('foo'), (getdate())).
• The collation was never copied from the old column: PostgreSQL only used the new definition's, and SQL Server looked it up as collate while getColumns() reports collation.
• compileChange() compiled every changed column for each change command.

Changes:

• The same approach as Fix changing a column on MySQL, MariaDB, PostgreSQL and SQL Server #258: copy the attributes the new definition doesn't set (nullable, default, the comment on PostgreSQL, and the collation while the column stays a text type) onto the changed column, and let Laravel's own compileChange() compile it. Unlike Fix changing a column on MySQL, MariaDB, PostgreSQL and SQL Server #258, the attributes are set on the column itself and not on a copy, because PostgreSQL compiles the comment and SQL Server the default in separate commands that run after the change.
• Keep the reported default as an Expression.
• Add tests with columns exactly as PostgreSQL and SQL Server report them. They fail without the change. The PostgreSQL test that asserted alter column "name" null now expects drop not null.
• Remove the two phpstan baseline entries for the old ColumnDefinition::$name access.

What it doesn't change: Laravel's own behavior stays. On PostgreSQL a change also emits drop identity if exists for a column that isn't auto-incrementing, and comment on column … is NULL when the column has no comment. On SQL Server a primary key column still can't be changed, so bigIncrements('id')->change() fails on the primary key constraint as it does without the shim. Winter 1.2 dropped the collation on PostgreSQL. This keeps it, as on MySQL.

This builds on #258 (and #254), and uses the existingColumns() test helper from #258, so it includes their commits. Only the last commit is new. The diff gets smaller once those are merged.

🤖 Generated with Claude Code

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[JonasPardon]

Closing in favour of #258, which now includes this change (e87c9fb).