[jira] [Commented] (HIVE-20198) Constant time table drops/renames

["Vihang Karajgaonkar (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-20198?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16743514#comment-16743514 ] 

Vihang Karajgaonkar commented on HIVE-20198:
--------------------------------------------

This somehow slipped through the cracks. I will assign it myself and see if I can take a first stab at it.

> Constant time table drops/renames
> ---------------------------------
>
>                 Key: HIVE-20198
>                 URL: https://issues.apache.org/jira/browse/HIVE-20198
>             Project: Hive
>          Issue Type: Improvement
>          Components: Metastore
>    Affects Versions: 4.0.0
>            Reporter: Alexander Kolbasov
>            Priority: Major
>
> Currently table drops and table renames have O(P) performance (where P is the number of partitions). When a managed table is deleted, the implementation deletes table metadata and then deletes all partitions in HDFS. HDFS operations are optimized and only do a sequential deletes for partitions outside of table prefix. This operation is O(P)where Pis the number of partitions. 
> Table rename goes through the list of partitions and modifies table name (and potentially db name) in each partition. It also modifies each partition location to match the new db/table name and renames directories (which is a non-atomic and slow operation on S3). This is O(P) operation where P is the number of partitions.
> Basic idea is to do the following:
> # Assign unique ID to each table
> # Create directory name based on unique ID rather then the name
> # Table rename then becomes metadata-only operation - there is no need to change any location information.
> # Table drop can become an asynchronous operation where the table is marked as "deleted". Subsequent public metadata APIs should skip such tables. A background cleaner thread may then go and clean up directories.
> Since the table location is unique for each table, new tables will not reuse existing locations. This change isn't compatible with the current behavior where there is an assumption that table location is based on table name. We can get around this by providing "opt-in" mechanism - special table property that tells that the table can have such new behavior, so the improvement will initially work for new tables created with this feature enabled. We may later provide some tool to convert existing tables to the new scheme.
> One complication is there in case where impersonation is enabled - the FS operations should be performed using client UGI rather then server's, so the cleaner thread should be able to use client UGIs.
> Initially we can punt on this and do standard table drops when impersonation is enabled.



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