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From "Mithun Radhakrishnan (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-7223) Support generic PartitionSpecs in Metastore partition-functions
Date Thu, 12 Jun 2014 05:04:01 GMT
Mithun Radhakrishnan created HIVE-7223:
------------------------------------------

             Summary: Support generic PartitionSpecs in Metastore partition-functions
                 Key: HIVE-7223
                 URL: https://issues.apache.org/jira/browse/HIVE-7223
             Project: Hive
          Issue Type: Improvement
          Components: HCatalog, Metastore
    Affects Versions: 0.13.0, 0.12.0
            Reporter: Mithun Radhakrishnan
            Assignee: Mithun Radhakrishnan


Currently, the functions in the HiveMetaStore API that handle multiple partitions do so using
List<Partition>. E.g. 
{code}
public List<Partition> listPartitions(String db_name, String tbl_name, short max_parts);
public List<Partition> listPartitionsByFilter(String db_name, String tbl_name, String
filter, short max_parts);
public int add_partitions(List<Partition> new_parts);
{code}

Partition objects are fairly heavyweight, since each Partition carries its own copy of a StorageDescriptor,
partition-values, etc. Tables with tens of thousands of partitions take so long to have their
partitions listed that the client times out with default hive.metastore.client.socket.timeout.
There is the additional expense of serializing and deserializing metadata for large sets of
partitions, w.r.t time and heap-space. Reducing the thrift traffic should help in this regard.

In a date-partitioned table, all sub-partitions for a particular date are *likely* (but not
expected) to have:

# The same base directory (e.g. {{/feeds/search/20140601/}})
# Similar directory structure (e.g. {{/feeds/search/20140601/[US,UK,IN]}})
# The same SerDe/StorageHandler/IOFormat classes
# Sorting/Bucketing/SkewInfo settings

In this “most likely” scenario (henceforth termed “normal”), it’s possible to represent
the partition-list (for a date) in a more condensed form: a list of LighterPartition instances,
all sharing a common StorageDescriptor whose location points to the root directory. 

We can go one better for the {{add_partitions()}} case: When adding all partitions for a given
date, the “normal” case affords us the ability to specify the top-level date-directory,
where sub-partitions can be inferred from the HDFS directory-path.

These extensions are hard to introduce at the metastore-level, since partition-functions explicitly
specify {{List<Partition>}} arguments. I wonder if a {{PartitionSpec}} interface might
help:

{{code}}
public PartitionSpec listPartitions(db_name, tbl_name, max_parts) throws ... ; 
public int add_partitions( PartitionSpec new_parts ) throws … ;
{{code}}

where the PartitionSpec looks like:

{{code}}
public interface PartitionSpec {
        public List<Partition> getPartitions();
        public List<String> getPartNames();
        public Iterator<Partition> getPartitionIter();
        public Iterator<String> getPartNameIter();
}
{{code}}

For addPartitions(), an {{HDFSDirBasedPartitionSpec}} class could implement {{PartitionSpec}},
store a top-level directory, and return Partition instances from sub-directory names, while
storing a single StorageDescriptor for all of them.

Similarly, list_partitions() could return a List<PartitionSpec>, where each PartitionSpec
corresponds to a set or partitions that can share a StorageDescriptor.

By exposing iterator semantics, neither the client nor the metastore need instantiate all
partitions at once. That should help with memory requirements.

In case no smart grouping is possible, we could just fall back on a {{DefaultPartitionSpec}}
which composes {{List<Partition>}}, and is no worse than status quo.

PartitionSpec abstracts away how a set of partitions may be represented. A tighter representation
allows us to communicate metadata for a larger number of Partitions, with less Thrift traffic.

Given that Thrift doesn’t support polymorphism, we’d have to implement the PartitionSpec
as a Thrift Union of supported implementations. (We could convert from the Thrift PartitionSpec
to the appropriate Java PartitionSpec sub-class.)

Thoughts?




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