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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-4287) Do lazy reading of parquet metadata cache file
Date Tue, 16 Feb 2016 19:58:18 GMT

    [ https://issues.apache.org/jira/browse/DRILL-4287?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15149203#comment-15149203
] 

ASF GitHub Bot commented on DRILL-4287:
---------------------------------------

Github user jinfengni commented on a diff in the pull request:

    https://github.com/apache/drill/pull/376#discussion_r53068818
  
    --- Diff: exec/java-exec/src/main/java/org/apache/drill/exec/store/parquet/ParquetGroupScan.java
---
    @@ -529,6 +549,36 @@ public long getRowCount() {
     
       }
     
    +
    +  // Create and return a new file selection based on reading the metadata cache file.
    +  // This function also initializes a few of ParquetGroupScan's fields as appropriate.
    +  private FileSelection
    +  initFromMetadataCache(DrillFileSystem fs, FileSelection selection) throws IOException
{
    +    FileStatus metaRootDir = selection.getFirstPath(fs);
    +    Path metaFilePath = new Path(metaRootDir.getPath(), Metadata.METADATA_FILENAME);
    +
    +    // get (and set internal field) the metadata for the directory by reading the metadata
file
    +    this.parquetTableMetadata = Metadata.readBlockMeta(fs, metaFilePath.toString());
    +    List<String> fileNames = Lists.newArrayList();
    +    for (Metadata.ParquetFileMetadata file : parquetTableMetadata.getFiles()) {
    +      fileNames.add(file.getPath());
    +    }
    +    // when creating the file selection, set the selection root in the form /a/b instead
of
    +    // file:/a/b.  The reason is that the file names above have been created in the form
    +    // /a/b/c.parquet and the format of the selection root must match that of the file
names
    +    // otherwise downstream operations such as partition pruning can break.
    +    final Path metaRootPath = Path.getPathWithoutSchemeAndAuthority(metaRootDir.getPath());
    +    this.selectionRoot = metaRootPath.toString();
    +
    +    // Use the FileSelection constructor directly here instead of the FileSelection.create()
method
    +    // because create() changes the root to include the scheme and authority; In future,
if create()
    +    // is the preferred way to instantiate a file selection, we may need to do something
different...
    +    FileSelection newSelection = new FileSelection(selection.getStatuses(fs), fileNames,
metaRootPath.toString());
    --- End diff --
    
    I see. Agreed that we should prioritize DRILL-4381 to address this inconsistency. 
    
    Overall, the revised patch looks good to me. 
    
    +1
     


> Do lazy reading of parquet metadata cache file
> ----------------------------------------------
>
>                 Key: DRILL-4287
>                 URL: https://issues.apache.org/jira/browse/DRILL-4287
>             Project: Apache Drill
>          Issue Type: Bug
>          Components: Query Planning & Optimization
>    Affects Versions: 1.4.0
>            Reporter: Aman Sinha
>            Assignee: Jinfeng Ni
>
> Currently, the parquet metadata cache file is read eagerly during creation of the DrillTable
(as part of ParquetFormatMatcher.isReadable()).  This is not desirable from performance standpoint
since there are scenarios where we want to do some up-front optimizations - e.g. directory-based
partition pruning (see DRILL-2517) or potential limit 0 optimization etc. - and in such situations
it is better to do lazy reading of the metadata cache file.   
> This is a placeholder to perform such delayed reading since it is needed for the aforementioned
optimizations.  



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