Björn Hegerfors created CASSANDRA-9420:
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Summary: Table option for promising that you will never touch a
column twice
Key: CASSANDRA-9420
URL: https://issues.apache.org/jira/browse/CASSANDRA-9420
Project: Cassandra
Issue Type: New Feature
Reporter: Björn Hegerfors
There are time series use cases where you write all values with various TTLs,
have GC grace = 0 and never ever update or delete a column after insertion. In
the case where all TTLs are the same, DTCS with recent patches works great. But
when there is lots of variations in TTLs, you are forced to choose between
splitting your table into multiple TTL tiers or having your SSTables filled to
the majority with tombstones. Or running frequent major compactions.
The problem stems from the fact that Cassandra plays safe when a TTL has
expired, and turns it into a tombstone, rather than getting rid of it on the
spot. The reason is that this TTL _may_ have been in a column which has had an
earlier write without (or with a higher) TTL. And then that one should now be
deleted too.
I propose that there should be table level setting to say "I guarantee that
there will never be any updates to any columns". The effect of enabling that
option is that all tombstones and expired TTLs should always be immediately
removed during compaction. And the check for dropping entirely expired SSTables
can be very loosened for these tables.
This option should probably require gc_grace_seconds to be set to zero. It's
also questionable if writes without TTL should be allowed to such a table,
since those would become constants.
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