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https://issues.apache.org/jira/browse/CASSANDRA-13992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16253591#comment-16253591
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Olivier Michallat commented on CASSANDRA-13992:
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[~KurtG]
bq. You wouldn't have to worry about METADATA_CHANGED being set because a LWT
will always have the same ID as the initial preparation
But then I would have to compare the ids every time. That costs me a lookup in
my client-side prepared statement cache (and there also happens to be an
additional volatile read in our current implementation). In contrast, checking
METADATA_CHANGED is free since it is already in the response.
> Don't send new_metadata_id for conditional updates
> --------------------------------------------------
>
> Key: CASSANDRA-13992
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13992
> Project: Cassandra
> Issue Type: Bug
> Reporter: Olivier Michallat
> Assignee: Kurt Greaves
> Priority: Minor
>
> This is a follow-up to CASSANDRA-10786.
> Given the table
> {code}
> CREATE TABLE foo (k int PRIMARY KEY)
> {code}
> And the prepared statement
> {code}
> INSERT INTO foo (k) VALUES (?) IF NOT EXISTS
> {code}
> The result set metadata changes depending on the outcome of the update:
> * if the row didn't exist, there is only a single column \[applied] = true
> * if it did, the result contains \[applied] = false, plus the current value
> of column k.
> The way this was handled so far is that the PREPARED response contains no
> result set metadata, and therefore all EXECUTE messages have SKIP_METADATA =
> false, and the responses always include the full (and correct) metadata.
> CASSANDRA-10786 still sends the PREPARED response with no metadata, *but the
> response to EXECUTE now contains a {{new_metadata_id}}*. The driver thinks it
> is because of a schema change, and updates its local copy of the prepared
> statement's result metadata.
> The next EXECUTE is sent with SKIP_METADATA = true, but the server appears to
> ignore that, and still sends the metadata in the response. So each response
> includes the correct metadata, the driver uses it, and there is no visible
> issue for client code.
> The only drawback is that the driver updates its local copy of the metadata
> unnecessarily, every time. We can work around that by only updating if we had
> metadata before, at the cost of an extra volatile read. But I think the best
> thing to do would be to never send a {{new_metadata_id}} in for a conditional
> update.
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