CREATE DATABASE statement
CREATE DATABASE statement is used to create a new database.
In Impala, a database is both:
- A logical construct for grouping together related tables, views, and functions within their own namespace. You might use a separate database for each application, set of related tables, or round of experimentation.
A physical construct represented by a directory tree in HDFS. Tables (internal tables), partitions, and
data files are all located under this directory. You can perform HDFS-level operations such as backing it up and measuring space usage,
or remove it with a
CREATE (DATABASE|SCHEMA) [IF NOT EXISTS] database_name[COMMENT 'database_comment'] [LOCATION hdfs_path];
Statement type: DDL
A database is physically represented as a directory in HDFS, with a filename extension
under the main Impala data directory. If the associated HDFS directory does not exist, it is created for you.
All databases and their associated directories are top-level objects, with no physical or logical nesting.
After creating a database, to make it the current database within an impala-shell session,
USE statement. You can refer to tables in the current database without prepending
any qualifier to their names.
When you first connect to Impala through impala-shell, the database you start in (before
CREATE DATABASE or
USE statements) is named
_impala_builtins, that serves as the location for the built-in functions. To see the built-in functions, use a statement like the following:
show functions in _impala_builtins; show functions in _impala_builtins like '*substring*';
After creating a database, your impala-shell session or another
impala-shell connected to the same node can immediately access that database. To access
the database through the Impala daemon on a different node, issue the
statement first while connected to that other node.
LOCATION attribute for a new database is a way to work with sets of files in an
HDFS directory structure outside the default Impala data directory, as opposed to setting the
LOCATION attribute for each individual table.
If you connect to different Impala nodes within an
impala-shell session for load-balancing purposes, you can enable the
SYNC_DDL query option to make each DDL statement wait before returning,
until the new or changed metadata has been received by all the Impala nodes.
When you create a database in Impala, the database can also be used by Hive.
When you create a database in Hive, issue an
statement in Impala to make Impala permanently aware of the new database.
SHOW DATABASES statement lists all databases, or the databases whose name
matches a wildcard pattern. In Impala 2.5 and higher, the
DATABASES output includes a second column that displays the associated comment,
if any, for each database.
Amazon S3 considerations:
To specify that any tables created within a database reside on
the Amazon S3 system, you can include an
s3a:// prefix on the
LOCATION attribute. In Impala 2.6 and higher, Impala automatically
creates any required folders as the databases, tables, and partitions are created, and
removes them when they are dropped.
In Impala 2.6 and higher, Impala DDL
statements such as
DROP DATABASE CASCADE,
DROP TABLE, and
TABLE [ADD|DROP] PARTITION can create or remove folders as needed in the Amazon
S3 system. Prior to Impala 2.6, you had to create folders yourself and point Impala
database, tables, or partitions at them, and manually remove folders when no longer needed.
Cancellation: Cannot be cancelled.
The user ID that the impalad daemon runs under,
impala user, must have write
permission for the parent HDFS directory under which the database
create database first_db; use first_db; create table t1 (x int); create database second_db; use second_db; -- Each database has its own namespace for tables. -- You can reuse the same table names in each database. create table t1 (s string); create database temp; -- You can either USE a database after creating it, -- or qualify all references to the table name with the name of the database. -- Here, tables T2 and T3 are both created in the TEMP database. create table temp.t2 (x int, y int); use database temp; create table t3 (s string); -- You cannot drop a database while it is selected by the USE statement. drop database temp; ERROR: AnalysisException: Cannot drop current default database: temp -- The always-available database 'default' is a convenient one to USE -- before dropping a database you created. use default; -- Before dropping a database, first drop all the tables inside it, -- or in Impala 2.3 and higher use the CASCADE clause. drop database temp; ERROR: ImpalaRuntimeException: Error making 'dropDatabase' RPC to Hive Metastore: CAUSED BY: InvalidOperationException: Database temp is not empty show tables in temp; +------+ | name | +------+ | t3 | +------+ -- Impala 2.3 and higher: drop database temp cascade; -- Earlier releases: drop table temp.t3; drop database temp;