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https://issues.apache.org/jira/browse/PHOENIX-5577?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16980834#comment-16980834
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Lars Hofhansl commented on PHOENIX-5577:
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[~comnetwork], maybe you have ideas?
> Uncovered columns are retrieved one-by-one in local index scans.
> ----------------------------------------------------------------
>
> Key: PHOENIX-5577
> URL: https://issues.apache.org/jira/browse/PHOENIX-5577
> Project: Phoenix
> Issue Type: Improvement
> Reporter: Lars Hofhansl
> Priority: Major
> Labels: performance
>
> One of the strengths of local indexes is that they are the only indexes that
> work when not all columns needed for a query are covered by (i.e. copied
> into) the index, allowing them to be *much* smaller. However the merging of
> the missing columns is done one-by-one per row.
> See RegionScannerFactory.getWrappedScanner(...) -> new
> RegionScanner(...).nextRaw(...) -> IndexUtil.wrapResultUsingOffset(...)
> For index scans this issues a Get back to the same region for each single
> scanned row. While the Get is local, it still needs to setup a scanner and
> seek to the right key each time. This is pretty inefficient. Local indexes
> could be much, much faster at read time for larger scans. This should use a
> SkipScan instead for a batch of scanned keys.
> (This is mitigated some by setting the block encoding to ROW_INDEX_V1, but
> still less than ideal.)
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