The purpose of `add_column_for_post_processing` is to add columns that are required for processing of a query,
but are not part of SELECT clause and shouldn't be returned. They are added to the final result set, but later are not serialized.
Mainly it is used for filtering and grouping columns, with a special case of `WHERE primary_key IN ... ORDER BY ...` when the whole result set needs additional final sorting,
and ordering columns must be added as well.
There was a bug that manifested in #9435, #8100 and was actually identified in #22061.
In case of selection with processing (e.g functions involved), result set row is formed in two stages.
Initially it is a list of columns fetched from replicas - on which filtering and grouping is performed.
After that the actual selection is resolved and the final number of columns can change.
Ordering is performed on this final shape, but the ordering column index returned by `add_column_for_post_processing` refereed to initial shape.
If selection refereed to the same column twice (e.g. `v, TTL(v)` as in #9435) final row was longer than initial and ordering refereed to incorrect column.
If a function in selection refereed to multiple columns (e.g. as_json(.., ..) which #8100 effectively uses) the final row was shorter
and ordering tried to use a non-existing column.
This patch fixes the problem by making sure that column index of the final result set is used for ordering.
The previously crashing test `cassandra_tests/validation/entities/json_test.py::testJsonOrdering` doesn't have to be skipped, but now it is failing on issue #28467.
Fixes#9435Fixes#8100Fixes#22061Closesscylladb/scylladb#28472
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