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Although Cassandra generally does not allow empty strings as partition keys (note they are allowed as clustering keys!), it *does* allow empty strings in regular columns to be indexed by a secondary index, or to become an empty partition-key column in a materialized view. As noted in issues #9375 and #9364 and verified in a few xfailing cql-pytest tests, Scylla didn't allow these cases - and this patch fixes that. The patch mostly *removes* unnecessary code: In one place, code prevented an sstable with an empty partition key from being written. Another piece of removed code was a function is_partition_key_empty() which the materialized-view code used to check whether the view's row will end up with an empty partition key, which was supposedly forbidden. But in fact, should have been allowed like they are allowed in Cassandra and required for the secondary-index implementation, and the entire function wasn't necessary. Note that the removed function is_partition_key_empty() was *NOT* required for the "IS NOT NULL" feature of materialized views - this continues to work as expected after this patch, and we add another test to confirm it. Being null and being an empty string are two different things. This patch also removes a part of a unit test which enshrined the wrong behavior. After this patch we are left with one interesting difference from Cassandra: Though Cassandra allows a user to create a view row with an empty-string partition key, and this row is fully visible in when scanning the view, this row can *not* be queried individually because "WHERE v=''" is forbidden when v is the partition key (of the view). Scylla does not reproduce this anomaly - and such point query does work in Scylla after this patch. We add a new test to check this case, and mark it "cassandra_bug", i.e., it's a Cassandra behavior which we consider wrong and don't want to emulate. This patch relies on #9352 and #10178 having been fixed in previous patches, otherwise the WHERE v='' does not work when reading from sstables. We add to the already existing tests we had for empty materialized-views keys a lookup with WHERE v='' which failed before fixing those two issues. Fixes #9364 Fixes #9375 Signed-off-by: Nadav Har'El <nyh@scylladb.com>