Support creating descending order indexes

[simonw]

SQLite lets you create indexes in reverse order, which can have a surprisingly big impact on performance, see simonw/covid-19-datasette#27

I tried doing this using sqlite-utils like so, but it's didn't work:

db["ny_times_us_counties"].create_index(["date desc"])

[simonw]

The problem here is differentiating between a column with the name date desc and wanting to create a descending index on a column called date.

This won't work:

db["ny_times_us_counties"].create_index(["date desc"], desc=True)

Because we need to be able to create compound indexes with columns with different directions - for example:

create index idx_age_desc_name on dogs (age desc, name)

[simonw]

A few options:

db["dogs"].create_index([("age", "desc"), "name"])

db["dogs"].create_index([desc("age"), "name"])

db["dogs"].create_index([db.desc("age"), "name"])

The first option uses an optional tuple. The second two use a desc() function - the question is where should that live?

sqlite_utils.desc(column) or db.desc(column) are both options.

I don't like using the term desc() for "descending index" though - it feels like it should mean something more broad.

[simonw]

Maybe:

db["dogs"].create_index([db.descending_index("age"), "name"])

It's a little verbose but it's for a relatively rare activity and it does make it very clear what is going on.

[simonw]

I could use db.desc_index("age") to match SQLite SQL syntax, which uses desc and not descending.

[simonw]

Need to solve this for the CLI tool too. Currently that works like this: https://sqlite-utils.datasette.io/en/stable/cli.html#creating-indexes

sqlite-utils create-index mydb.db mytable col1 [col2...]

Even harder to decide how to add a descending option to this. Maybe like this?

sqlite-utils create-index mydb.db mytable --direction col1 asc --direction col2 desc

It's a bit gross though! We're saying here that if a single one of the columns you are creating an index for is in reverse direction you have to use --direction to specify each end every other index.

[simonw]

For the CLI version I could say that you can use a - prefix to specify reverse direction:

sqlite-utils create-index mydb.db dogs -age name

No, that doesn't work - it could get confused with a command-line flag. I guess you could do this:

sqlite-utils create-index mydb.db dogs "-age" name

This does mean that if any of your column names begin with a hyphen you can't use the CLI to add indexes to them. Is that an acceptable limitation? Users can always use sqlite-utils mydb.db "create index ..." in that case.

[simonw]

Decisions: for the Python API I'm going with db.DescIndex("column") as the way to do this.

For the CLI I'm going to do the "-age" thing.

[simonw]

Annoyingly the table.indexes property won't indicate if an index is in regular or reverse order - because the SQLite PRAGMA index_info(table) statement doesn't indicate that either. You have to look at the sqlite_master index definition to tell if any of the columns are in reverse order:

(Pdb) fresh_db.execute("select * from sqlite_master where type = 'index'").fetchall()
[('index', 'idx_dogs_age_name', 'dogs', 3, 'CREATE INDEX [idx_dogs_age_name]\n    ON [dogs] ([age] desc, [name])')]
(Pdb) fresh_db.execute("PRAGMA index_info('idx_dogs_age_name')").fetchall()
[(0, 2, 'age'), (1, 0, 'name')]
(Pdb) fresh_db.execute("PRAGMA index_info('idx_dogs_age_name')").description
(('seqno', None, None, None, None, None, None), ('cid', None, None, None, None, None, None), ('name', None, None, None, None, None, None))
(Pdb) dogs.indexes
[Index(seq=0, name='idx_dogs_age_name', unique=0, origin='c', partial=0, columns=['age', 'name'])]

[simonw]

PRAGMA index_xinfo(table) DOES return that data:

(Pdb) [c[0] for c in fresh_db.execute("PRAGMA index_xinfo('idx_dogs_age_name')").description]
['seqno', 'cid', 'name', 'desc', 'coll', 'key']
(Pdb) fresh_db.execute("PRAGMA index_xinfo('idx_dogs_age_name')").fetchall()
[(0, 2, 'age', 1, 'BINARY', 1), (1, 0, 'name', 0, 'BINARY', 1), (2, -1, None, 0, 'BINARY', 0)]

See https://sqlite.org/pragma.html#pragma_index_xinfo

Example output: https://covid-19.datasettes.com/covid?sql=select+*+from+pragma_index_xinfo%28%27idx_ny_times_us_counties_date%27%29

[simonw]

It's weird having to use Database.DescIndex - I'm going to put DescIndex in sqlite_utils.db directly and let people import it.