.transform() fails to drop column if table is part of a view

[simonw]

I got this error trying to drop a column from a table that was part of a SQL view:

error in view plugins: no such table: main.pypi_releases

Upon further investigation I found that this pattern seemed to fix it:

def transform_the_table(conn):
    # Run this in a transaction:
    with conn:
        # We have to read all the views first, because we need to drop and recreate them
        db = sqlite_utils.Database(conn)
        views = {v.name: v.schema for v in db.views if table.lower() in v.schema.lower()}
        for view in views.keys():
            db[view].drop()
        db[table].transform(
            types=types,
            rename=rename,
            drop=drop,
            column_order=[p[0] for p in order_pairs],
        )
        # Now recreate the views
        for name, schema in views.items():
            db.create_view(name, schema)

So grab a copy of any view that might reference this table, start a transaction, drop those views, run the transform, recreate the views again.

I wonder if this should become an option in sqlite-utils? Maybe a recreate_views=True argument for table.tranform(...)? Should it be opt-in or opt-out?

Originally posted by @simonw in simonw/datasette-edit-schema#35 (comment)

[simonw]

More notes in here:

• Error if you try to transform a table that is part of a view datasette-edit-schema#35 (comment)

Not all Python/SQLite installations exhibit this problem by default!

It turns out this is controlled by the legacy_alter_table pragma: https://sqlite.org/pragma.html#pragma_legacy_alter_table

If that PRAGMA is turned on (default in newer SQLites) then alter table will error if you try to rename a table that is referenced in a view.

Here's a one-liner to test if it is on or not:

python -c 'import sqlite3; print(sqlite3.connect(":memory:").execute("PRAGMA legacy_alter_table").fetchall())'

[simonw]

Options:

• Provide a recreate_views: bool parameter to table.transform() controlling if views that might reference this table are stashed and dropped and recreated within a transaction as part of the operation. But should that be True or False by default?
• Read that PRAGMA and automatically do that view workaround if it's turned on
• Toggle that PRAGMA off for the duration of the .transform() operation and on again at the end. Does it only affect the current connection?
• Try the transform() in a transaction, detect the "error in view", "no such table"error, if spotted then do the VIEW workaround and try again

I'm on the fence as to which of these I like the most. I'm tempted to go with the one which just drops VIEWS and recreates them all the time, because it feels simpler.

[simonw]

I shipped the view recreating fix in datasette-edit-schema, so at least I can start exercising that fix and see if it has any weird issues.

[tobych]

Just noting that the fix @simonw mentions is here in datasette-edit-schema.

I'm using views that reference tables that are changing, so I'm likely to want this fix.

[tobych]

The documentation for the legacy_alter_table pragma states that the behavior it controls is per-connection, and not persistent.

Regardless, using the pragma seems a bad idea to me, because:

• the feature it's controlling is a legacy behavior
• the documentation says: "New applications should leave this flag turned off."

The recreate_views idea seems like it might be useful, but I reckon You Ain't Gonna Need It. If someone has a lot of views, and it takes ages to recreate them, perhaps. That seems highly unlikely.