INVALIDATE METADATA Statement
Marks the metadata for one or all tables as stale. Required after a table is created through the Hive shell, before the table is available for Impala queries. The next time the current Impala node performs a query against a table whose metadata is invalidated, Impala reloads the associated metadata before the query proceeds. This is a relatively expensive operation compared to the incremental metadata update done by the REFRESH statement, so in the common scenario of adding new data files to an existing table, prefer REFRESH rather than INVALIDATE METADATA. If you are not familiar with the way Impala uses metadata and how it shares the same metastore database as Hive, see Overview of Impala Metadata and the Metastore for background information.
INVALIDATE METADATA [[db_name.]table_name]
By default, the cached metadata for all tables is flushed. If you specify a table name, only the metadata for that one table is flushed. Even for a single table, INVALIDATE METADATA is more expensive than REFRESH, so prefer REFRESH in the common case where you add new data files for an existing table.
To accurately respond to queries, Impala must have current metadata about those databases and tables that clients query directly. Therefore, if some other entity modifies information used by Impala in the metastore that Impala and Hive share, the information cached by Impala must be updated. However, this does not mean that all metadata updates require an Impala update.
INVALIDATE METADATA and REFRESH are counterparts: INVALIDATE METADATA waits to reload the metadata when needed for a subsequent query, but reloads all the metadata for the table, which can be an expensive operation, especially for large tables with many partitions. REFRESH reloads the metadata immediately, but only loads the block location data for newly added data files, making it a less expensive operation overall. If data was altered in some more extensive way, such as being reorganized by the HDFS balancer, use INVALIDATE METADATA to avoid a performance penalty from reduced local reads. If you used Impala version 1.0, the INVALIDATE METADATA statement works just like the Impala 1.0 REFRESH statement did, while the Impala 1.1 REFRESH is optimized for the common use case of adding new data files to an existing table, thus the table name argument is now required.
A metadata update for an impalad instance is required if:
- A metadata change occurs.
- and the change is made from another impalad instance in your cluster, or through Hive.
- and the change is made to a metastore database to which clients such as the Impala shell or ODBC directly connect.
A metadata update for an Impala node is not required when you issue queries from the same Impala node where you ran ALTER TABLE, INSERT, or other table-modifying statement.
Database and table metadata is typically modified by:
- Hive - via ALTER, CREATE, DROP or INSERT operations.
- Impalad - via CREATE TABLE, ALTER TABLE, and INSERT operations.
INVALIDATE METADATA causes the metadata for that table to be marked as stale, and reloaded the next time the table is referenced. For a huge table, that process could take a noticeable amount of time; thus you might prefer to use REFRESH where practical, to avoid an unpredictable delay later, for example if the next reference to the table is during a benchmark test.
The following example shows how you might use the INVALIDATE METADATA statement after creating new tables (such as SequenceFile or HBase tables) through the Hive shell. Before the INVALIDATE METADATA statement was issued, Impala would give a "table not found" error if you tried to refer to those table names. The DESCRIBE statements cause the latest metadata to be immediately loaded for the tables, avoiding a delay the next time those tables are queried.
[impalad-host:21000] > invalidate metadata; [impalad-host:21000] > describe t1; ... [impalad-host:21000] > describe t2; ...
For more examples of using REFRESH and INVALIDATE METADATA with a combination of Impala and Hive operations, see Switching Back and Forth Between Impala and Hive.
If you need to ensure that the metadata is up-to-date when you start an impala-shell session, run impala-shell with the -r or --refresh_after_connect command-line option. Because this operation adds a delay to the next query against each table, potentially expensive for large tables with many partitions, try to avoid using this option for day-to-day operations in a production environment.
The user ID that the impalad daemon runs under, typically the impala user, must have execute permissions for all the relevant directories holding table data. (A table could have data spread across multiple directories, or in unexpected paths, if it uses partitioning or specifies a LOCATION attribute for individual partitions or the entire table.) Issues with permissions might not cause an immediate error for this statement, but subsequent statements such as SELECT or SHOW TABLE STATS could fail.
By default, the INVALIDATE METADATA command checks HDFS permissions of the underlying data files and directories, caching this information so that a statement can be cancelled immediately if for example the impala user does not have permission to write to the data directory for the table. (This checking does not apply if you have set the catalogd configuration option --load_catalog_in_background=false.) Impala reports any lack of write permissions as an INFO message in the log file, in case that represents an oversight. If you change HDFS permissions to make data readable or writeable by the Impala user, issue another INVALIDATE METADATA to make Impala aware of the change.
This example illustrates creating a new database and new table in Hive, then doing an INVALIDATE METADATA statement in Impala using the fully qualified table name, after which both the new table and the new database are visible to Impala. The ability to specify INVALIDATE METADATA table_name for a table created in Hive is a new capability in Impala 1.2.4. In earlier releases, that statement would have returned an error indicating an unknown table, requiring you to do INVALIDATE METADATA with no table name, a more expensive operation that reloaded metadata for all tables and databases.
$ hive hive> create database new_db_from_hive; OK Time taken: 4.118 seconds hive> create table new_db_from_hive.new_table_from_hive (x int); OK Time taken: 0.618 seconds hive> quit; $ impala-shell [localhost:21000] > show databases like 'new*'; [localhost:21000] > refresh new_db_from_hive.new_table_from_hive; ERROR: AnalysisException: Database does not exist: new_db_from_hive [localhost:21000] > invalidate metadata new_db_from_hive.new_table_from_hive; [localhost:21000] > show databases like 'new*'; +--------------------+ | name | +--------------------+ | new_db_from_hive | +--------------------+ [localhost:21000] > show tables in new_db_from_hive; +---------------------+ | name | +---------------------+ | new_table_from_hive | +---------------------+
Amazon S3 considerations:
The REFRESH and INVALIDATE METADATA statements also cache metadata for tables where the data resides in the Amazon Simple Storage Service (S3). In particular, issue a REFRESH for a table after adding or removing files in the associated S3 data directory. See Using Impala with the Amazon S3 Filesystem for details about working with S3 tables.
Cancellation: Cannot be cancelled.
Much of the metadata for Kudu tables is handled by the underlying storage layer. Kudu tables have less reliance on the metastore database, and require less metadata caching on the Impala side. For example, information about partitions in Kudu tables is managed by Kudu, and Impala does not cache any block locality metadata for Kudu tables.
The REFRESH and INVALIDATE METADATA statements are needed less frequently for Kudu tables than for HDFS-backed tables. Neither statement is needed when data is added to, removed, or updated in a Kudu table, even if the changes are made directly to Kudu through a client program using the Kudu API. Run REFRESH table_name or INVALIDATE METADATA table_name for a Kudu table only after making a change to the Kudu table schema, such as adding or dropping a column, by a mechanism other than Impala.