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https://issues.apache.org/jira/browse/CASSANDRA-6870?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13941885#comment-13941885
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Jonathan Ellis commented on CASSANDRA-6870:
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Let's back up a bit.
We're trying to support
{{update table set col1=increment(col1), col2=trim(col2) where rowkey = 5}}
or generalized to {{update table set col3=pow(col1, col2), col4=trim(col5)
where rowkey = 5}}
I infer a couple things from this:
- this should support normal CL conventions. So if I want to trim without
paxos because I'm confident that there are no races, I shouldn't have to pay
that extra latency cost. I think this means we open up CL.SERIAL for normal
UPDATE (without IF) when UDF are being invoked against data from an existing
row.
- paxos always reads the existing values at QUORUM, but it looks like we will
want to allow a separate CL to be specified for reads as well
- we need to be able to combine multiple function calls / transformations into
a single update / paxos round. I *think* this is straightforward but I may be
wrong.
- sometimes a single output column is not enough. how do we support more? We
could use {{(c1, c2) = f(c3, c4)}} syntax similar to what we introduced for
composite partition keys. Or we could add "out" parameters which is what most
SQL UDF implementations do. NB: I am fine with postponing implementation of
this to another ticket, but I do think we should settle on the syntax ahead of
time so that we don't accidentally make extending it harder than necessary.
> Transform operation
> -------------------
>
> Key: CASSANDRA-6870
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6870
> Project: Cassandra
> Issue Type: New Feature
> Reporter: Edward Capriolo
> Assignee: Edward Capriolo
> Priority: Minor
>
> Compare and swap uses paxos to only update a value only if some criteria is
> met. If I understand correctly we should be able to use this feature to
> provide a wider variety of server side operations.
> For example inside a paxos transaction performing a slice and then using a
> function to manipulate the slice. You could accomplish features like append
> and increment this way without user needing to know the current value.
> I took a stab at doing this. I **think** I did it correctly. Comments welcome.
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