[jira] [Commented] (HIVE-11611) A bad performance regression issue with Parquet happens if Hive does not select any columns

["Cheng Lian (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-11611?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14706312#comment-14706312 ] 

Cheng Lian commented on HIVE-11611:
-----------------------------------

PARQUET-363 provided more background and details about this issue.

> A bad performance regression issue with Parquet happens if Hive does not select any columns
> -------------------------------------------------------------------------------------------
>
>                 Key: HIVE-11611
>                 URL: https://issues.apache.org/jira/browse/HIVE-11611
>             Project: Hive
>          Issue Type: Sub-task
>    Affects Versions: 2.0.0
>            Reporter: Sergio Peña
>            Assignee: Ferdinand Xu
>         Attachments: HIVE-11611.patch
>
>
> A possible performance issue may happen with the below code when using a query like this {{SELECT count(1) FROM parquetTable}}.
> {code}
> if (!ColumnProjectionUtils.isReadAllColumns(configuration) && !indexColumnsWanted.isEmpty()) {
>         MessageType requestedSchemaByUser =
>             getSchemaByIndex(tableSchema, columnNamesList, indexColumnsWanted);
>         return new ReadContext(requestedSchemaByUser, contextMetadata);
> } else {
>   return new ReadContext(tableSchema, contextMetadata);
> }
> {code}
> If there are not columns nor indexes selected, then the above code will read the full schema from Parquet even if Hive does not do anything with such values.



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