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https://issues.apache.org/jira/browse/HIVE-14792?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mithun Radhakrishnan updated HIVE-14792:
----------------------------------------
    Status: Patch Available  (was: Open)

Submitting, to run tests.

> AvroSerde reads the remote schema-file at least once per mapper, per table 
> reference.
> -------------------------------------------------------------------------------------
>
>                 Key: HIVE-14792
>                 URL: https://issues.apache.org/jira/browse/HIVE-14792
>             Project: Hive
>          Issue Type: Bug
>    Affects Versions: 2.1.0, 1.2.1
>            Reporter: Mithun Radhakrishnan
>            Assignee: Mithun Radhakrishnan
>         Attachments: HIVE-14792.1.patch
>
>
> Avro tables that use "external" schema files stored on HDFS can cause 
> excessive calls to {{FileSystem::open()}}, especially for queries that spawn 
> large numbers of mappers.
> This is because of the following code in {{AvroSerDe::initialize()}}:
> {code:title=AvroSerDe.java|borderStyle=solid}
> public void initialize(Configuration configuration, Properties properties) 
> throws SerDeException {
> // ...
>     if (hasExternalSchema(properties)
>         || columnNameProperty == null || columnNameProperty.isEmpty()
>         || columnTypeProperty == null || columnTypeProperty.isEmpty()) {
>       schema = determineSchemaOrReturnErrorSchema(configuration, properties);
>     } else {
>       // Get column names and sort order
>       columnNames = Arrays.asList(columnNameProperty.split(","));
>       columnTypes = 
> TypeInfoUtils.getTypeInfosFromTypeString(columnTypeProperty);
>       schema = getSchemaFromCols(properties, columnNames, columnTypes, 
> columnCommentProperty);
>          
> properties.setProperty(AvroSerdeUtils.AvroTableProperties.SCHEMA_LITERAL.getPropName(),
>  schema.toString());
>     }
> // ...
> }
> {code}
> For tables using {{avro.schema.url}}, every time the SerDe is initialized 
> (i.e. at least once per mapper), the schema file is read remotely. For 
> queries with thousands of mappers, this leads to a stampede to the handful 
> (3?) datanodes that host the schema-file. In the best case, this causes 
> slowdowns.
> It would be preferable to distribute the Avro-schema to all mappers as part 
> of the job-conf. The alternatives aren't exactly appealing:
> # One can't rely solely on the {{column.list.types}} stored in the Hive 
> metastore. (HIVE-14789).
> # {{avro.schema.literal}} might not always be usable, because of the 
> size-limit on table-parameters. The typical size of the Avro-schema file is 
> between 0.5-3MB, in my limited experience. Bumping the max table-parameter 
> size isn't a great solution.
> If the {{avro.schema.file}} were read during query-planning, and made 
> available as part of table-properties (but not serialized into the 
> metastore), the downstream logic will remain largely intact. I have a patch 
> that does this.



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