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https://issues.apache.org/jira/browse/IMPALA-15157?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Quanlong Huang reassigned IMPALA-15157:
---------------------------------------
Assignee: YifanZhang
> Parquet scanner unnecessarily decompresses a fully-skipped page at a page
> boundary
> ----------------------------------------------------------------------------------
>
> Key: IMPALA-15157
> URL: https://issues.apache.org/jira/browse/IMPALA-15157
> Project: IMPALA
> Issue Type: Improvement
> Reporter: YifanZhang
> Assignee: YifanZhang
> Priority: Major
> Fix For: Impala 5.0.0
>
>
> When late materialization / row-level skipping skips a set of rows that ends
> exactly on a Parquet page boundary,
> {{BaseScalarColumnReader::SkipRowsInternal()}} exits the page-skipping loop
> and calls {{{}ReadCurrentDataPage(){}}}, decompressing that final page just
> to skip all of its values. The whole page could instead be discarded without
> decompression.
> *Root cause*
> The whole-page-skip loop discarded pages only while the number of rows to
> skip was _strictly greater_ than the values buffered in the current page:
> {code:java}
> while (num_rows > num_buffered_values_) {
> ...
> if (!col_chunk_reader_.SkipPageData().ok() || !AdvanceNextPageHeader()) {
> return false;
> }
> } {code}
> When {{num_rows == num_buffered_values_}} (the skip ends exactly at a page
> boundary), the loop exited and the code proceeded to decompress the current
> page and skip values in-page. In the boundary case where nothing remains
> after the current page, this path still calls {{AdvanceNextPageHeader()}} for
> a page header that may not exist, and needlessly decompresses a page that is
> going to be fully skipped.
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