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https://issues.apache.org/jira/browse/IMPALA-5036?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Taras Bobrovytsky resolved IMPALA-5036.
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Resolution: Fixed
Fix Version/s: Impala 2.10.0
> Improve COUNT(*) performance of Parquet scans.
> ----------------------------------------------
>
> Key: IMPALA-5036
> URL: https://issues.apache.org/jira/browse/IMPALA-5036
> Project: IMPALA
> Issue Type: Sub-task
> Components: Backend
> Affects Versions: Impala 2.5.0, Impala 2.6.0, Impala 2.7.0, Impala 2.8.0
> Reporter: Alexander Behm
> Assignee: Taras Bobrovytsky
> Labels: parquet, performance, ramp-up
> Fix For: Impala 2.10.0
>
>
> {code}
> select count(*) from parquet_table;
> select count(*) from parquet_table group by partition_col;
> {code}
> Impala already has a special code path for fast Parquet scans when no columns
> are scanned and materialized, but the performance can be significantly
> improved with a plan+execution change, as follows:
> *Execution change*
> Instead of returning empty batches until num_rows have been returned, the
> Parquet scanner can populate a single slot with the num_rows from the Parquet
> row groups
> *Plan change*
> The count(*) local aggregation needs to be changed to a sum(num_rows_slot)
> aggregation.
> The final distributed plan will be:
> scan -> local agg with sum(num_rows_slot) -> merge agg sum(sum(num_rows_slot))
> This optimization is applicable where is only a count(*) and there are no
> scan predicates.
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