DDL refers to "Data Definition Language", a subset of SQL statements that change the structure of the database schema in some way, typically by creating, deleting, or modifying schema objects such as databases, tables, and views. Most Impala DDL statements start with the keywords CREATE, DROP, or ALTER.
The Impala DDL statements are:
- ALTER TABLE Statement
- ALTER VIEW Statement
- COMPUTE STATS Statement
- CREATE DATABASE Statement
- CREATE FUNCTION Statement
- CREATE ROLE Statement (CDH 5.2 or higher only)
- CREATE TABLE Statement
- CREATE VIEW Statement
- DROP DATABASE Statement
- DROP FUNCTION Statement
- DROP ROLE Statement (CDH 5.2 or higher only)
- DROP TABLE Statement
- DROP VIEW Statement
- GRANT Statement (CDH 5.2 or higher only)
- REVOKE Statement (CDH 5.2 or higher only)
After Impala executes a DDL command, information about available tables, columns, views, partitions, and so on is automatically synchronized between all the Impala nodes in a cluster. (Prior to Impala 1.2, you had to issue a REFRESH or INVALIDATE METADATA statement manually on the other nodes to make them aware of the changes.)
If the timing of metadata updates is significant, for example if you use round-robin scheduling where each query could be issued through a different Impala node, you can enable the SYNC_DDL query option to make the DDL statement wait until all nodes have been notified about the metadata changes.
See Using Impala to Query the Amazon S3 Filesystem for details about how Impala DDL statements interact with tables and partitions stored in the Amazon S3 filesystem.
Although the INSERT statement is officially classified as a DML (data manipulation language) statement, it also involves metadata changes that must be broadcast to all Impala nodes, and so is also affected by the SYNC_DDL query option.
Because the SYNC_DDL query option makes each DDL operation take longer than normal, you might only enable it before the last DDL operation in a sequence. For example, if you are running a script that issues multiple of DDL operations to set up an entire new schema, add several new partitions, and so on, you might minimize the performance overhead by enabling the query option only before the last CREATE, DROP, ALTER, or INSERT statement. The script only finishes when all the relevant metadata changes are recognized by all the Impala nodes, so you could connect to any node and issue queries through it.
The classification of DDL, DML, and other statements is not necessarily the same between Impala and Hive. Impala organizes these statements in a way intended to be familiar to people familiar with relational databases or data warehouse products. Statements that modify the metastore database, such as COMPUTE STATS, are classified as DDL. Statements that only query the metastore database, such as SHOW or DESCRIBE, are put into a separate category of utility statements.