Mithun Radhakrishnan created HIVE-7223:
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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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