Impala Describe Statement – Syntax, Usage & Example
It is possible to displays metadata about a table in Impala. For that, we use the Impala Describe Statement. So, in this article, we will discuss, how to use Impala Describe Statement. Also, we will cover its syntax, usage as well as Example to understand it well.
So, let’s start Impala Describe Statement.
How to use Impala Describe Statement?
In order to displays metadata about a table, we use the Impala Describe Statement. Like the column names and their data types.
Basically, in the format familiar to users of Apache Hive, the DESCRIBE FORMATTED variation displays additional information. It includes low-level details.
Such as whether the table is internal or external when it was created, the file format, the location of the data in HDFS, whether the object is a table or a view, and (for views) the text of the query from the view definition.
To be more specific, to give the description of the table, we use the Impala Describe Statement. It contains the information about a table like the column names and their data types.
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Also, the abbreviation for the DESCRIBE statement is.
Syntax of Impala Describe Statements
So, the syntax for using Impala Describe Statement is-
DESCRIBE [FORMATTED] [db_name.]table_name[.complex_col_name ...]
Usage of Impala Describe Statements
However, the first query against a table can take longer than subsequent queries, just after the impalad daemons are restarted. It happens because the metadata for the table is loaded before the query is processed.
Although, if the delay occurs even one-time delay for each table it cause misleading results in benchmark tests or cause unnecessary concern. Hence, we can issue a DESCRIBE statement in advance for each table we intend to access later to “warm up” the Impala metadata cache.
Sometimes it is important to know details such as the path of the data files for an Impala table, and the hostname for the namenode, when we are dealing with data files stored in HDFS. It is possible to get this information from the DESCRIBE FORMATTED output.
Hence, we can easily specify HDFS URIs or path specifications with statements. Like LOAD DATA and the LOCATION clause of CREATE TABLE or ALTER TABLE. Also, we might use HDFS URIs or paths with Linux commands. Like Hadoop and Hdfs to copy, rename, and so on, data files in HDFS.
In addition, every table can have associated table statistics and column statistics with them. we can use the SHOW TABLE STATS table_name and SHOW COLUMN STATS table_name statements to see these categories of information.
Example of Impala Describe Statements
Let’s suppose we have a table named Employees in Impala. Here, is the data −
Further, by using the describe statement, we can get the description of the Employees table
[quickstart.cloudera:21000] > describe Employees;
So, Impala fetches the metadata of the specified table, by executing the above query.
Query: describe Employees
Fetched 5 row(s) in 0.51s
Describing the Records using Hue
There are several steps you have to follow while Describing the records using Hue. Such as:
- At first, type the Describe Statement in Impala Query editor. Further, click on the execute button.
- Afterward, we can see the metadata of the table once you scroll down and select the Results tab just after executing the query.
It is not possible to cancel it. That implies it Cannot be canceled.
HDFS permissions in Impala
It is must that the Impala user ID has read and executed permissions for all directories that are part of the table that the impalad daemon runs under.
So, this was all in Impala Describe Statements. Hope you like our explanation.
Conclusion – Impala Describe Statements
Hence, we have seen the whole concept of Impala Describe Statement. Still, if any doubt occurs in how to use Impala Describe Statement, feel free to ask in the comment section.