T-SQL CreateIndex.where() is omitted for filtered indexes

[cofin]

Summary

CreateIndex.where() is available on the SQLSpec DDL builder, but the predicate is not emitted when rendering a SQL Server / T-SQL index. This means filtered unique indexes still need raw SQL even though the builder already models both if_not_exists() and where().

Environment

• SQLSpec: 0.54.2
• Python: 3.14.4
• Dialect: tsql

Reproduction

from sqlspec import sql

stmt = (
    sql.create_index("ix_queue_tasks_task_key")
    .if_not_exists()
    .unique()
    .on_table("queue_tasks")
    .columns("task_key")
    .where("task_key IS NOT NULL")
)

print(stmt.build(dialect="tsql").sql)

Actual output

IF NOT EXISTS (SELECT * FROM sys.indexes WHERE object_id = object_id('queue_tasks') AND name = 'ix_queue_tasks_task_key') EXEC('CREATE UNIQUE INDEX [ix_queue_tasks_task_key] ON [queue_tasks]([task_key])')

The T-SQL IF NOT EXISTS ... EXEC(...) guard is rendered, but the filtered-index predicate is missing. Rendering without if_not_exists() also omits the predicate:

stmt = (
    sql.create_index("ix_queue_tasks_task_key")
    .unique()
    .on_table("queue_tasks")
    .columns("task_key")
    .where("task_key IS NOT NULL")
)
print(stmt.build(dialect="tsql").sql)

Actual:

CREATE UNIQUE INDEX [ix_queue_tasks_task_key] ON [queue_tasks]([task_key])

Expected output

For T-SQL, the generated index DDL should include the predicate, for example:

IF NOT EXISTS (SELECT * FROM sys.indexes WHERE object_id = object_id('queue_tasks') AND name = 'ix_queue_tasks_task_key') EXEC('CREATE UNIQUE INDEX [ix_queue_tasks_task_key] ON [queue_tasks]([task_key]) WHERE [task_key] IS NOT NULL')

For the non-guarded case:

CREATE UNIQUE INDEX [ix_queue_tasks_task_key] ON [queue_tasks]([task_key]) WHERE [task_key] IS NOT NULL

Use case

litestar-queues needs a nullable unique task key on SQL Server. SQL Server requires a filtered unique index (WHERE task_key IS NOT NULL) to allow multiple rows with NULL task keys while enforcing uniqueness for keyed tasks. Because the T-SQL builder drops .where(...), that backend still has to carry hand-written raw T-SQL for this DDL.

[cofin]

T-SQL filtered index predicate omission appears to come from SQLGlot index rendering. SQLSpec should not add a custom T-SQL workaround here; keeping this open for upstream tracking unless a future fix is a generic AST placement change across dialects.