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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-10560) Remove unnecessary index lookups for single partition reads
Date Wed, 21 Oct 2015 13:08:27 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-10560?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14966777#comment-14966777
] 

Sylvain Lebresne commented on CASSANDRA-10560:
----------------------------------------------

I'd actually kind of prefer not doing that, because I don't think this provides benefits and
it's remotely risky.

I don't think it provides benefits because we're not really doing a lookup. The fact we don't
use {{SinglePartitionReadCommand}} for indexes means that the {{RowFilter}} will be empty
and in that case, {{SecondaryIndexManager.getBestIndexFor()}} exits right away. One exception
might be filtering queries, which might have a non empty {{RowFilter}} but with nothing indexed,
but I'm not sure we should worry about this very small cost when {{ALLOW FILTERING}} is involved
in the first place.

It's also potentially risky because while {{SelectStatement}} currently indeed never uses
{{SinglePartitionReadCommand}}, nothing forbid other internal queries to do so. I'm sure no
internal query currently uses that, but it would be easy to mess up. It's not reasonable to
expect that everyone contributing will know that {{SinglePartitionReadCommand}} seemingly
randomly (and silently) skips indexes. At a minimum, this might make whomever changes {{SelectStatement}}
so it stops avoiding {{SinglePartitionReadCommand}} for index scratch his head for a long
time to figure out why this doesn't work (and there is absolutely no good reason not to use
{{SinglePartitionReadCommand}} for indexes, so we should do this change very soon).

> Remove unnecessary index lookups for single partition reads
> -----------------------------------------------------------
>
>                 Key: CASSANDRA-10560
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10560
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Sam Tunnicliffe
>            Assignee: Sam Tunnicliffe
>             Fix For: 3.0.0
>
>
> For single partition reads, we currently perform an unnecessary call to SecondaryIndexManager
to lookup the most selective index based on the command's RowFilter. This is redundant as
if any applicable indexes are present, the query would be performed using a range command
(there was some code introduced into {{CassandraIndexSearcher}} by CASSANDRA-8099 in preparation
for supporting indexes on single partition reads, but it is as yet unused). Given that, we
should make sure we don't perform that lookup for now.



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