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https://issues.apache.org/jira/browse/IMPALA-15394?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Quanlong Huang updated IMPALA-15394:
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Epic Link: IMPALA-14596
> HBO stores count(*) scans under the key of a regular scan
> ---------------------------------------------------------
>
> Key: IMPALA-15394
> URL: https://issues.apache.org/jira/browse/IMPALA-15394
> Project: IMPALA
> Issue Type: Bug
> Components: Frontend
> Reporter: Aleksandr Efimov
> Assignee: Aleksandr Efimov
> Priority: Major
>
> An HdfsScanNode with the count\(*) optimization returns one row per file or
> row group, so its cardinality is the file count.
> HdfsScanNode.generateHboKeyString() does not include countStarSlot_, so HBO
> stores this run under the same key as a regular scan of the same table and
> conjuncts. The next regular scan takes the file count as its cardinality.
> Seen on a Parquet table with 15 files and 13.66M rows: after {{SELECT
> count\(*) FROM t}}, {{SELECT * FROM t}} is planned with {{cardinality=15
> (from HBO)}}. The memory estimates above the scan follow it, and so does the
> per-query memory limit when admission control derives it from the estimate. A
> regular scan that runs later fixes the record, because the last run wins.
> Reproduce: on a Parquet table with more than one file and no HBO record, run
> {{SELECT count\(*) FROM t}}, then {{EXPLAIN SELECT * FROM t}}.
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