The Impala Query Language and Built-in Functions - Learning Cloudera Impala - page 78

Query-specific SQL statements in

Impala

Now, we will spend some time in understanding the query-specific SQL statements
used in Impala. Most of these statements are exactly the same as they are defined in
SQL, so to learn more, I would suggest you to look at any SQL reference document-
ation. Here, I am covering some key information for reference purposes:

• EXPLAIN : Using the EXPLAIN clause, we can learn the execution plan of a
SQL statement by understanding low-level mechanisms that Impala will use
to read and process the data in the whole cluster, and then finally show the
results. You can use the EXPLAIN clause ahead of a SELECT statement as
shown in the following example:

[Hadoop.testdomain:21000] > EXPLAIN SELECT
* FROM list;
Explain query: select * from list
PLAN FRAGMENT 0
PARTITION: UNPARTITIONED

1:EXCHANGE
tuple ids: 0

PLAN FRAGMENT 1
PARTITION: RANDOM

STREAM DATA SINK
EXCHANGE ID: 1
UNPARTITIONED

0:SCAN HDFS
table=default.list #partitions=0
size=0B
tuple ids: 0
REFRESH table_name;