impala/docs/topics/impala_truncate_table.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept rev="2.3.0" id="truncate_table">

  <title>TRUNCATE TABLE Statement (<keyword keyref="impala23"/> or higher only)</title>
  <titlealts audience="PDF"><navtitle>TRUNCATE TABLE</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="DDL"/>
      <data name="Category" value="S3"/>
      <data name="Category" value="Tables"/>
      <data name="Category" value="Disk Storage"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p rev="2.3.0">
      <indexterm audience="Cloudera">TRUNCATE TABLE statement</indexterm>
      Removes the data from an Impala table while leaving the table itself.
    </p>

    <p conref="../shared/impala_common.xml#common/syntax_blurb"/>

<codeblock>TRUNCATE TABLE <ph rev="2.5.0 IMPALA-2641">[IF EXISTS]</ph> [<varname>db_name</varname>.]<varname>table_name</varname></codeblock>

    <p conref="../shared/impala_common.xml#common/ddl_blurb"/>

    <p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>

    <p>
      Often used to empty tables that are used during ETL cycles, after the data has been copied to another
      table for the next stage of processing. This statement is a low-overhead alternative to dropping and
      recreating the table, or using <codeph>INSERT OVERWRITE</codeph> to replace the data during the
      next ETL cycle.
    </p>

    <p>
      This statement removes all the data and associated data files in the table. It can remove data files from internal tables,
      external tables, partitioned tables, and tables mapped to HBase or the Amazon Simple Storage Service (S3).
      The data removal applies to the entire table, including all partitions of a partitioned table.
    </p>

    <p>
      Any statistics produced by the <codeph>COMPUTE STATS</codeph> statement are reset when the data is removed.
    </p>

    <p>
      Make sure that you are in the correct database before truncating a table, either by issuing a
      <codeph>USE</codeph> statement first or by using a fully qualified name
      <codeph><varname>db_name</varname>.<varname>table_name</varname></codeph>.
    </p>

    <p rev="2.5.0 IMPALA-2641">
      The optional <codeph>IF EXISTS</codeph> clause makes the statement succeed whether or not the table exists.
      If the table does exist, it is truncated; if it does not exist, the statement has no effect. This capability is
      useful in standardized setup scripts that are might be run both before and after some of the tables exist.
      This clause is available in <keyword keyref="impala25_full"/> and higher.
    </p>

    <p>
      Any HDFS data files removed by this statement go into the HDFS trashcan, from which you can recover them
      within a defined time interval if this operation turns out to be a mistake.
    </p>

    <p conref="../shared/impala_common.xml#common/disk_space_blurb"/>

    <p conref="../shared/impala_common.xml#common/s3_blurb"/>
    <p rev="2.2.0">
      Although Impala cannot write new data to a table stored in the Amazon
      S3 filesystem, the <codeph>TRUNCATE TABLE</codeph> statement can remove data files from S3.
      See <xref href="impala_s3.xml#s3"/> for details about working with S3 tables.
    </p>

    <p conref="../shared/impala_common.xml#common/cancel_blurb_no"/>

    <p conref="../shared/impala_common.xml#common/permissions_blurb"/>
    <p rev="CDH-19187">
      The user ID that the <cmdname>impalad</cmdname> daemon runs under,
      typically the <codeph>impala</codeph> user, must have write
      permission for all the files and directories that make up the table.
    </p>

    <p conref="../shared/impala_common.xml#common/example_blurb"/>

    <p>
      The following example shows a table containing some data and with table and column statistics.
      After the <codeph>TRUNCATE TABLE</codeph> statement, the data is removed and the statistics
      are reset.
    </p>

<codeblock>CREATE TABLE truncate_demo (x INT);
INSERT INTO truncate_demo VALUES (1), (2), (4), (8);
SELECT COUNT(*) FROM truncate_demo;
+----------+
| count(*) |
+----------+
| 4        |
+----------+
COMPUTE STATS truncate_demo;
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 1 column(s). |
+-----------------------------------------+
SHOW TABLE STATS truncate_demo;
+-------+--------+------+--------------+-------------------+--------+-------------------+
| #Rows | #Files | Size | Bytes Cached | Cache Replication | Format | Incremental stats |
+-------+--------+------+--------------+-------------------+--------+-------------------+
| 4     | 1      | 8B   | NOT CACHED   | NOT CACHED        | TEXT   | false             |
+-------+--------+------+--------------+-------------------+--------+-------------------+
SHOW COLUMN STATS truncate_demo;
+--------+------+------------------+--------+----------+----------+
| Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+------+------------------+--------+----------+----------+
| x      | INT  | 4                | -1     | 4        | 4        |
+--------+------+------------------+--------+----------+----------+

-- After this statement, the data and the table/column stats will be gone.
TRUNCATE TABLE truncate_demo;

SELECT COUNT(*) FROM truncate_demo;
+----------+
| count(*) |
+----------+
| 0        |
+----------+
SHOW TABLE STATS truncate_demo;
+-------+--------+------+--------------+-------------------+--------+-------------------+
| #Rows | #Files | Size | Bytes Cached | Cache Replication | Format | Incremental stats |
+-------+--------+------+--------------+-------------------+--------+-------------------+
| -1    | 0      | 0B   | NOT CACHED   | NOT CACHED        | TEXT   | false             |
+-------+--------+------+--------------+-------------------+--------+-------------------+
SHOW COLUMN STATS truncate_demo;
+--------+------+------------------+--------+----------+----------+
| Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------+------+------------------+--------+----------+----------+
| x      | INT  | -1               | -1     | 4        | 4        |
+--------+------+------------------+--------+----------+----------+
</codeblock>

    <p>
      The following example shows how the <codeph>IF EXISTS</codeph> clause allows the <codeph>TRUNCATE TABLE</codeph>
      statement to be run without error whether or not the table exists:
    </p>

<codeblock rev="2.5.0 IMPALA-2641">CREATE TABLE staging_table1 (x INT, s STRING);
Fetched 0 row(s) in 0.33s

SHOW TABLES LIKE 'staging*';
+----------------+
| name           |
+----------------+
| staging_table1 |
+----------------+
Fetched 1 row(s) in 0.25s

-- Our ETL process involves removing all data from several staging tables
-- even though some might be already dropped, or not created yet.

TRUNCATE TABLE IF EXISTS staging_table1;
Fetched 0 row(s) in 5.04s

TRUNCATE TABLE IF EXISTS staging_table2;
Fetched 0 row(s) in 0.25s

TRUNCATE TABLE IF EXISTS staging_table3;
Fetched 0 row(s) in 0.25s
</codeblock>

    <p conref="../shared/impala_common.xml#common/related_info"/>

    <p>
      <xref href="impala_tables.xml#tables"/>,
      <xref href="impala_alter_table.xml#alter_table"/>, <xref href="impala_create_table.xml#create_table"/>,
      <xref href="impala_partitioning.xml#partitioning"/>, <xref href="impala_tables.xml#internal_tables"/>,
      <xref href="impala_tables.xml#external_tables"/>
    </p>

  </conbody>
</concept>