Use CREATE INDEX with DROP_EXISTING=ON for index facet changes (SQL Server)

[m-x-shokhzod]

When an index facet changes (fill factor, sort order, uniqueness, filter, columns), MigrationsModelDiffer produces a DropIndexOperation + CreateIndexOperation pair, which the SQL Server generator emits as two statements:

DROP INDEX [IX_People_X] ON [People];
CREATE UNIQUE INDEX [IX_People_X] ON [People] ([X]);

This is suboptimal: during the drop, queries can't use the index → potentially full table scans, and on large tables the gap can be significant.

SQL Server supports CREATE INDEX ... WITH (DROP_EXISTING = ON) since SQL Server 2008, which:

• Atomically replaces the old index in a single statement
• Lets queries continue using the old index while the new one is being built — no un-indexed gap

After

CREATE UNIQUE INDEX [IX_People_X] ON [People] ([X]) WITH (DROP_EXISTING = ON);

Implementation

The rewrite lives entirely in the SQL Server provider:

• New internal annotation SqlServerAnnotationNames.UseDropExisting
• New helper SqlServerMigrationsSqlGenerator.RewriteDropAndCreateIndexAsDropExisting (called from RewriteOperations, alongside the existing FixLegacyTemporalAnnotations) — detects Drop+Create pairs for the same (Name, Table, Schema), removes the drop from the operation list, and marks the create with UseDropExisting = true
• IndexOptions reads the annotation and adds DROP_EXISTING = ON to the WITH (...) clause alongside FILLFACTOR, ONLINE, SORT_IN_TEMPDB, DATA_COMPRESSION

Limited to standard indexes — memory-optimized, full-text, and vector indexes use different syntax/restrictions and fall back to the existing drop+create path.

MigrationsModelDiffer is unchanged; the differ still emits Drop+Create as before. Other providers (MySQL, PostgreSQL, SQLite, Cosmos) are unaffected.

Tests

Test	Type	Verifies
Alter_index_make_unique	functional (override)	Existing test — SQL expectation updated to single-statement form
Alter_index_change_sort_order	functional (override)	Existing test — SQL expectation updated
Alter_index_fill_factor_uses_drop_existing	functional (new)	FILLFACTOR change collapses to WITH (FILLFACTOR = 90, DROP_EXISTING = ON)
Alter_index_filter_uses_drop_existing	functional (new)	Filter change collapses to WITH (DROP_EXISTING = ON), filter rendered after WHERE

All 4 tests pass end-to-end against a real SQL Server instance (verified locally against Azure SQL Edge on Apple Silicon ARM64; the CI matrix will additionally verify against SqlServer 2019/2022/2025).

Full regression sweep: EFCore.SqlServer.Tests 1335/1335, SqlServerMigrationsSqlGeneratorTest 124/124, MigrationsSqlServerTest index tests 34/34.

Fixes #35067