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https://issues.apache.org/jira/browse/DRILL-4530?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15352088#comment-15352088
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ASF GitHub Bot commented on DRILL-4530:
---------------------------------------
Github user amansinha100 commented on a diff in the pull request:
https://github.com/apache/drill/pull/519#discussion_r68678168
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/store/dfs/FileSelection.java
---
@@ -47,16 +47,25 @@
private List<FileStatus> statuses;
public List<String> files;
+ /**
+ * root path for the selections
+ */
public final String selectionRoot;
+ /**
+ * root path for the metadata cache file (if any)
+ */
+ public final String cacheFileRoot;
--- End diff --
That was my initial approach (updating the selectionRoot without keeping a
separate cacheFileRoot). However, I ran into a few issues. The main one
that I recall is that the dir0, dir1 etc columns are associated with the
selectionRoot, so suppose I run the following query:
SELECT dir0, dir1 FROM dfs.tmp.t2 WHERE dir0=2015 AND dir1='Q1'
and if the selectionRoot gets modified to point to '2015/Q1' then we have
lost the context of the original dir0, dir1 because everything will become
relative to the new selectionRoot. This produces wrong results. The same
problem occurred with a SELECT * query where the directory columns where not
showing up correctly.
> Improve metadata cache performance for queries with single partition
> ---------------------------------------------------------------------
>
> Key: DRILL-4530
> URL: https://issues.apache.org/jira/browse/DRILL-4530
> Project: Apache Drill
> Issue Type: Improvement
> Components: Query Planning & Optimization
> Affects Versions: 1.6.0
> Reporter: Aman Sinha
> Assignee: Aman Sinha
> Fix For: Future
>
>
> Consider two types of queries which are run with Parquet metadata caching:
> {noformat}
> query 1:
> SELECT col FROM `A/B/C`;
> query 2:
> SELECT col FROM `A` WHERE dir0 = 'B' AND dir1 = 'C';
> {noformat}
> For a certain dataset, the query1 elapsed time is 1 sec whereas query2
> elapsed time is 9 sec even though both are accessing the same amount of data.
> The user expectation is that they should perform roughly the same. The main
> difference comes from reading the bigger metadata cache file at the root
> level 'A' for query2 and then applying the partitioning filter. query1 reads
> a much smaller metadata cache file at the subdirectory level.
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