CREATE DATABASE statement
The 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
DROP DATABASE
statement.
Syntax:
CREATE (DATABASE|SCHEMA) [IF NOT EXISTS] database_name[COMMENT 'database_comment']
[LOCATION hdfs_path];
Statement type: DDL
Usage notes:
A database is physically represented as a directory in HDFS, with a filename extension .db
,
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 the 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
issuing any CREATE DATABASE
or USE
statements) is named
default
.
_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 INVALIDATE METADATA
statement first while connected to that other node.
Setting the 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.
Hive considerations:
When you create a database in Impala, the database can also be used by Hive.
When you create a database in Hive, issue an INVALIDATE METADATA
statement in Impala to make Impala permanently aware of the new database.
The SHOW DATABASES
statement lists all databases, or the databases whose name
matches a wildcard pattern. In Impala 2.5 and higher, the SHOW
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 CREATE DATABASE
, CREATE TABLE
,
DROP DATABASE CASCADE
, DROP TABLE
, and ALTER
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.
HDFS permissions:
The user ID that the impalad daemon runs under,
typically the impala
user, must have write
permission for the parent HDFS directory under which the database
is located.
Examples:
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;