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If you generate a pickle using Django version N, there is no guarantee that pickle will be readable with Django version N 1.
If a query doesn’t have an ordering specified, results are returned from the database in an unspecified order.This means that when you unpickle a attribute is an opaque object.It represents the internals of the query construction and is not part of the public API.The above example will return the last item first, then the penultimate item and so on. If you order by fields from a related model, those fields will be added to the selected columns and they may make otherwise duplicate rows appear to be distinct.Since the extra columns don’t appear in the returned results (they are only there to support ordering), it sometimes looks like non-distinct results are being returned.You can order by a field converted to lowercase with Warning Ordering is not a free operation.
Each field you add to the ordering incurs a cost to your database.
You can evaluate a , this will force all the results to be loaded into memory prior to pickling.
Pickling is usually used as a precursor to caching and when the cached queryset is reloaded, you want the results to already be present and ready for use (reading from the database can take some time, defeating the purpose of caching).
To put it simply: if you need to ask, you don’t need to use it. An expression may be a simple value, a reference to a field on the model (or any related models), or an aggregate expression (averages, sums, etc.) that has been computed over the objects that are related to the objects in the that is returned.
The aggregation functions that are provided by Django are described in Aggregation Functions below.
can be constructed, filtered, sliced, and generally passed around without actually hitting the database.