===========
Limitations
===========

Available filter expressions
----------------------------

Most but not all filter expressions that django-filter supports are supported.
To see a list of available expressions for each property filter see :ref:`filter-reference`


Property types
--------------

Because properties are evaluated at runtime the types cannot be predetermined
beforehand like it is the case with database fields.
Therefore there might be unexpected behaviour during filtering.


Performance
-----------

Because all the filtering is done in memory it performs slower than django-filter
where the filtering happens directly in sql.
This will be impacted by the numbers of filters used at the same time and the
size of the data in the table.

Database limitations
--------------------

    In theory there is no limit for most databases how many results can be returned
    from a filter query unless the database implements a limit which will impact how
    many results django-property-filter can return.

    **sqlite**

    .. warning::
        Sqlite3 defines SQLITE_MAX_VARIABLE_NUMBER which is a limit for parameters
        passed to a query.

        See "Maximum Number Of Host Parameters In A Single SQL Statement" at
        https://www.sqlite.org/limits.html for further details.

        Depending on the version this limit might differ.
        By default from version 3.32.0 onwards, sqlite should have a default of 32766 while
        versions before this the limit was 999.
        A different limit can also be set at compile time and python is compiling their own sqlite version.

        For example Python 3.9.1 comes with sqlite version 3.33.0 and the 999 max parameter limitation still exists

    Because of the way django-property-filter queries the database (i.e. with a prefilterd list of primary keys),
    the number of sql parameters needed might exceed the set limit.

    Django-property-filter will try to return all values if possible, but if not
    possible it will try to return as many as possible limiting the sql parameters to not more than 999
    and log a warning message
    similar to::
        WARNING:root:Only returning the first 3746 items because of max parameter limitations of Database "sqlite"

    It is possible to set a custom limit via the environment variable
    "USER_DB_MAX_PARAMS". For example the user uses a custom compiled sqlite
    version with a different than the default value for SQLITE_MAX_VARIABLE_NUMBER
    the setting "USER_DB_MAX_PARAMS" to that value will use this value as a
    fallback rather than default values.