impala/docs/topics/impala_refresh.xml

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

  <title>REFRESH Statement</title>
  <titlealts audience="PDF"><navtitle>REFRESH</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="DDL"/>
      <data name="Category" value="Tables"/>
      <data name="Category" value="Hive"/>
      <data name="Category" value="Metastore"/>
      <data name="Category" value="ETL"/>
      <data name="Category" value="Ingest"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="Cloudera">REFRESH statement</indexterm>
      To accurately respond to queries, the Impala node that acts as the coordinator (the node to which you are
      connected through <cmdname>impala-shell</cmdname>, JDBC, or ODBC) must have current metadata about those
      databases and tables that are referenced in Impala queries. If you are not familiar with the way Impala uses
      metadata and how it shares the same metastore database as Hive, see
      <xref href="impala_hadoop.xml#intro_metastore"/> for background information.
    </p>

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

<codeblock rev="IMPALA-1683 CDH-43732">REFRESH [<varname>db_name</varname>.]<varname>table_name</varname> [PARTITION (<varname>key_col1</varname>=<varname>val1</varname> [, <varname>key_col2</varname>=<varname>val2</varname>...])]</codeblock>

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

    <p>
      Use the <codeph>REFRESH</codeph> statement to load the latest metastore metadata and block location data for
      a particular table in these scenarios:
    </p>

    <ul>
      <li>
        After loading new data files into the HDFS data directory for the table. (Once you have set up an ETL
        pipeline to bring data into Impala on a regular basis, this is typically the most frequent reason why
        metadata needs to be refreshed.)
      </li>

      <li>
        After issuing <codeph>ALTER TABLE</codeph>, <codeph>INSERT</codeph>, <codeph>LOAD DATA</codeph>, or other
        table-modifying SQL statement in Hive.
      </li>
    </ul>

    <note rev="2.3.0">
      <p rev="2.3.0">
        In <keyword keyref="impala23_full"/> and higher, the syntax <codeph>ALTER TABLE <varname>table_name</varname> RECOVER PARTITIONS</codeph>
        is a faster alternative to <codeph>REFRESH</codeph> when the only change to the table data is the addition of
        new partition directories through Hive or manual HDFS operations.
        See <xref href="impala_alter_table.xml#alter_table"/> for details.
      </p>
    </note>

    <p>
      You only need to issue the <codeph>REFRESH</codeph> statement on the node to which you connect to issue
      queries. The coordinator node divides the work among all the Impala nodes in a cluster, and sends read
      requests for the correct HDFS blocks without relying on the metadata on the other nodes.
    </p>

    <p>
      <codeph>REFRESH</codeph> reloads the metadata for the table from the metastore database, and does an
      incremental reload of the low-level block location data to account for any new data files added to the HDFS
      data directory for the table. It is a low-overhead, single-table operation, specifically tuned for the common
      scenario where new data files are added to HDFS.
    </p>

    <p>
      Only the metadata for the specified table is flushed. The table must already exist and be known to Impala,
      either because the <codeph>CREATE TABLE</codeph> statement was run in Impala rather than Hive, or because a
      previous <codeph>INVALIDATE METADATA</codeph> statement caused Impala to reload its entire metadata catalog.
    </p>

    <note>
      <p rev="1.2">
        The catalog service broadcasts any changed metadata as a result of Impala
        <codeph>ALTER TABLE</codeph>, <codeph>INSERT</codeph> and <codeph>LOAD DATA</codeph> statements to all
        Impala nodes. Thus, the <codeph>REFRESH</codeph> statement is only required if you load data through Hive
        or by manipulating data files in HDFS directly. See <xref href="impala_components.xml#intro_catalogd"/> for
        more information on the catalog service.
      </p>
      <p rev="1.2.1">
        Another way to avoid inconsistency across nodes is to enable the
        <codeph>SYNC_DDL</codeph> query option before performing a DDL statement or an <codeph>INSERT</codeph> or
        <codeph>LOAD DATA</codeph>.
      </p>
      <p rev="1.1">
        The table name is a required parameter. To flush the metadata for all tables, use the
        <codeph><xref href="impala_invalidate_metadata.xml#invalidate_metadata">INVALIDATE METADATA</xref></codeph>
        command.
      </p>
      <p conref="../shared/impala_common.xml#common/invalidate_then_refresh"/>
    </note>

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

    <p>
      A metadata update for an <codeph>impalad</codeph> instance <b>is</b> required if:
    </p>

    <ul>
      <li>
        A metadata change occurs.
      </li>

      <li>
        <b>and</b> the change is made through Hive.
      </li>

      <li>
        <b>and</b> the change is made to a metastore database to which clients such as the Impala shell or ODBC directly
        connect.
      </li>
    </ul>

    <p rev="1.2">
      A metadata update for an Impala node is <b>not</b> required after you run <codeph>ALTER TABLE</codeph>,
      <codeph>INSERT</codeph>, or other table-modifying statement in Impala rather than Hive. Impala handles the
      metadata synchronization automatically through the catalog service.
    </p>

    <p>
      Database and table metadata is typically modified by:
    </p>

    <ul>
      <li>
        Hive - through <codeph>ALTER</codeph>, <codeph>CREATE</codeph>, <codeph>DROP</codeph> or
        <codeph>INSERT</codeph> operations.
      </li>

      <li>
        Impalad - through <codeph>CREATE TABLE</codeph>, <codeph>ALTER TABLE</codeph>, and <codeph>INSERT</codeph>
        operations. <ph rev="1.2">Such changes are propagated to all Impala nodes by the
        Impala catalog service.</ph>
      </li>
    </ul>

    <p>
      <codeph>REFRESH</codeph> causes the metadata for that table to be immediately reloaded. For a huge table,
      that process could take a noticeable amount of time; but doing the refresh up front avoids an unpredictable
      delay later, for example if the next reference to the table is during a benchmark test.
    </p>

    <p rev="IMPALA-1683 CDH-43732">
      <b>Refreshing a single partition:</b>
    </p>

    <p rev="IMPALA-1683 CDH-43732">
      In <keyword keyref="impala27_full"/> and higher, the <codeph>REFRESH</codeph> statement can apply to a single partition at a time,
      rather than the whole table. Include the optional <codeph>PARTITION (<varname>partition_spec</varname>)</codeph>
      clause and specify values for each of the partition key columns.
    </p>

    <p rev="IMPALA-1683 CDH-43732">
      The following examples show how to make Impala aware of data added to a single partition, after data is loaded into
      a partition's data directory using some mechanism outside Impala, such as Hive or Spark. The partition can be one that
      Impala created and is already aware of, or a new partition created through Hive.
    </p>

<codeblock rev="IMPALA-1683 CDH-43732"><![CDATA[
impala> create table p (x int) partitioned by (y int);
impala> insert into p (x,y) values (1,2), (2,2), (2,1);
impala> show partitions p;
+-------+-------+--------+------+...
| y     | #Rows | #Files | Size |...
+-------+-------+--------+------+...
| 1     | -1    | 1      | 2B   |...
| 2     | -1    | 1      | 4B   |...
| Total | -1    | 2      | 6B   |...
+-------+-------+--------+------+...

-- ... Data is inserted into one of the partitions by some external mechanism ...
beeline> insert into p partition (y = 1) values(1000);

impala> refresh p partition (y=1);
impala> select x from p where y=1;
+------+
| x    |
+------+
| 2    | <- Original data created by Impala
| 1000 | <- Additional data inserted through Beeline
+------+
]]>
</codeblock>

    <p rev="IMPALA-1683 CDH-43732">
      The same applies for tables with more than one partition key column.
      The <codeph>PARTITION</codeph> clause of the <codeph>REFRESH</codeph>
      statement must include all the partition key columns.
    </p>

<codeblock rev="IMPALA-1683 CDH-43732"><![CDATA[
impala> create table p2 (x int) partitioned by (y int, z int);
impala> insert into p2 (x,y,z) values (0,0,0), (1,2,3), (2,2,3);
impala> show partitions p2;
+-------+---+-------+--------+------+...
| y     | z | #Rows | #Files | Size |...
+-------+---+-------+--------+------+...
| 0     | 0 | -1    | 1      | 2B   |...
| 2     | 3 | -1    | 1      | 4B   |...
| Total |   | -1    | 2      | 6B   |...
+-------+---+-------+--------+------+...

-- ... Data is inserted into one of the partitions by some external mechanism ...
beeline> insert into p2 partition (y = 2, z = 3) values(1000);

impala> refresh p2 partition (y=2, z=3);
impala> select x from p where y=2 and z = 3;
+------+
| x    |
+------+
| 1    | <- Original data created by Impala
| 2    | <- Original data created by Impala
| 1000 | <- Additional data inserted through Beeline
+------+
]]>
</codeblock>

    <p rev="IMPALA-1683 CDH-43732">
      The following examples show how specifying a nonexistent partition does not cause any error,
      and the order of the partition key columns does not have to match the column order in the table.
      The partition spec must include all the partition key columns; specifying an incomplete set of
      columns does cause an error.
    </p>

<codeblock rev="IMPALA-1683 CDH-43732"><![CDATA[
-- Partition doesn't exist.
refresh p2 partition (y=0, z=3);
refresh p2 partition (y=0, z=-1)
-- Key columns specified in a different order than the table definition.
refresh p2 partition (z=1, y=0)
-- Incomplete partition spec causes an error.
refresh p2 partition (y=0)
ERROR: AnalysisException: Items in partition spec must exactly match the partition columns in the table definition: default.p2 (1 vs 2)
]]>
</codeblock>

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

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

    <p>
      The following example shows how you might use the <codeph>REFRESH</codeph> statement after manually adding
      new HDFS data files to the Impala data directory for a table:
    </p>

<codeblock>[impalad-host:21000] &gt; refresh t1;
[impalad-host:21000] &gt; refresh t2;
[impalad-host:21000] &gt; select * from t1;
...
[impalad-host:21000] &gt; select * from t2;
... </codeblock>

    <p>
      For more examples of using <codeph>REFRESH</codeph> and <codeph>INVALIDATE METADATA</codeph> with a
      combination of Impala and Hive operations, see <xref href="impala_tutorial.xml#tutorial_impala_hive"/>.
    </p>

    <p>
      <b>Related impala-shell options:</b>
    </p>

    <p rev="1.1">
      The <cmdname>impala-shell</cmdname> option <codeph>-r</codeph> issues an <codeph>INVALIDATE METADATA</codeph> statement
      when starting up the shell, effectively performing a <codeph>REFRESH</codeph> of all tables.
      Due to the expense of reloading the metadata for all tables, the <cmdname>impala-shell</cmdname> <codeph>-r</codeph>
      option is not recommended for day-to-day use in a production environment. (This option was mainly intended as a workaround
      for synchronization issues in very old Impala versions.)
    </p>

    <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 execute
      permissions for all the relevant directories holding table data.
      (A table could have data spread across multiple directories,
      or in unexpected paths, if it uses partitioning or
      specifies a <codeph>LOCATION</codeph> attribute for
      individual partitions or the entire table.)
      Issues with permissions might not cause an immediate error for this statement,
      but subsequent statements such as <codeph>SELECT</codeph>
      or <codeph>SHOW TABLE STATS</codeph> could fail.
    </p>
    <p rev="IMPALA-1683 CDH-43732">
      All HDFS and Sentry permissions and privileges are the same whether you refresh the entire table
      or a single partition.
    </p>

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

    <p>
      The <codeph>REFRESH</codeph> command checks HDFS permissions of the underlying data files and directories,
      caching this information so that a statement can be cancelled immediately if for example the
      <codeph>impala</codeph> user does not have permission to write to the data directory for the table. Impala
      reports any lack of write permissions as an <codeph>INFO</codeph> message in the log file, in case that
      represents an oversight. If you change HDFS permissions to make data readable or writeable by the Impala
      user, issue another <codeph>REFRESH</codeph> to make Impala aware of the change.
    </p>

    <note conref="../shared/impala_common.xml#common/compute_stats_next"/>

    <p conref="../shared/impala_common.xml#common/s3_blurb"/>
    <p conref="../shared/impala_common.xml#common/s3_metadata"/>

    <p conref="../shared/impala_common.xml#common/cancel_blurb_no"/>
    <p conref="../shared/impala_common.xml#common/related_info"/>
    <p>
      <xref href="impala_hadoop.xml#intro_metastore"/>,
      <xref href="impala_invalidate_metadata.xml#invalidate_metadata"/>
    </p>
  </conbody>
</concept>