[ https://issues.apache.org/jira/browse/HIVE-14792?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Mithun Radhakrishnan updated HIVE-14792: ---------------------------------------- Status: Patch Available (was: Reopened) > 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 > Labels: TODOC2.2, TODOC2.4 > Fix For: 3.0.0, 2.4.0, 2.2.1 > > Attachments: HIVE-14792.1.patch, HIVE-14792.3.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. -- This message was sent by Atlassian JIRA (v6.4.14#64029)