YifanZhang created IMPALA-15157:
-----------------------------------
Summary: Parquet scanner reads a non-existent page header when
late materialization skips rows ending exactly on a page boundary
Key: IMPALA-15157
URL: https://issues.apache.org/jira/browse/IMPALA-15157
Project: IMPALA
Issue Type: Improvement
Reporter: YifanZhang
{{BaseScalarColumnReader::SkipRowsInternal()}} (in
{{{}be/src/exec/parquet/parquet-column-readers.cc{}}}) can attempt to read a
page header that does not exist when late materialization / row-level skipping
needs to skip a set of rows that ends exactly on a Parquet page boundary — for
example, when an entire row group is filtered out by row-level predicates and
the rows to skip run to the end of the column chunk.
*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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