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From Björn Hegerfors (JIRA) <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-9420) Table option for promising that you will never touch a column twice
Date Mon, 18 May 2015 21:23:00 GMT
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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