MySQL: cannot add key limited index on TEXT columns

[Tobias Krönke]

Heyo!

I have a model with a text field url = models.TextField() and I would like to add an index to my MySQL 8.0.36 DB to speed up queries like url__startswith='https://google.com%' -> WHERE url LIKE 'https://google.com%'. I can actually do it by creating an index manually like so:

CREATE INDEX `my_limited_prefix_index` ON `my_model_table` (url(100));

However, I would like to be able to do it with the ORM. Having it only in the migrations as a data migration has many downsides:

• cannot run tests without running the migrations if i need the index in tests
• makes squashing migrations more painful

But it seems, this is impossible to achieve with the Meta.indexes:

Index(fields=['url(100)'], name='my_limited_prefix_index')

-> denied by django with (models.E012) 'indexes' refers to the nonexistent field 'url(100)'

Index(expressions.RawSQL('url(100)', ()), name='my_limited_prefix_index')

-> denied by MySQL with syntax error (see ​https://github.com/sqlalchemy/sqlalchemy/issues/5462, django adds double parantheses which is only valid syntax for expressions, but here I need a column definition)

Index(expressions.RawSQL('LEFT(url,100)', ()), name='my_limited_prefix_index')

-> creates a useless index that cannot be used by MySQL for startswith (aka not sargable anymore).

I guess my favourite solution would be to allow the 2nd way with being able to turn off the surrounding expression parantheses.

Thx!

[Sarah Boyce]

I feel this functionality exists and the way to go about this might be using ​Index.expressions and ​Left

[Tobias Krönke]

Replying to Sarah Boyce:

I feel this functionality exists and the way to go about this might be using ​Index.expressions and ​Left

Thx. Unfortunately, your feeling is not accurate. Your suggestion results in the same index created as my 3rd approach in the OP. That index however is useless. It cannot be used by MySQL for speeding up startswith queries.

[Tobias Krönke]

This is the kind of index I would like to be able to create: ​https://dev.mysql.com/doc/refman/8.4/en/create-index.html#create-index-column-prefixes. So called Column Prefix Key Parts.

[Sarah Boyce]

​Index.condition with Q(startswith='https://google.com') is probably also an option.

[Tobias Krönke]

The url__startswith='https://google.com%' query was just an example. I would like to optimize all startswith queries on that field. I've created this crude workaround and implemented my own Index:

class ColumnPrefixIndex:
    max_name_length = 60
    contains_expressions = False
    fields_orders = []
    include = []
    expressions = []
    fields = []
    condition = None

    def __init__(self, column, prefix, name):
        self.prefix = prefix
        self.column = column
        self.name = name

    def create_sql(self, model, schema_editor, using="", **kwargs):
        table = model._meta.db_table
        sql = f'CREATE INDEX {self.name} ON {table} ({self.column}({self.prefix}));'
        kwargs['sql'] = sql

        return schema_editor._create_index_sql(
            model,
            name=self.name,
            using=using,
            **kwargs,
        )

    def deconstruct(self):
        kwargs = {'column': self.column, 'prefix': self.prefix, 'name': self.name}
        path = "%s.%s" % (self.__class__.__module__, self.__class__.__name__)
        return path, self.expressions, kwargs

    def clone(self):
        return self.__class__(self.column, self.prefix, self.name)