impala/docs/topics/impala_seqfile.xml

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

  <title id="sequencefile">Using the SequenceFile File Format with Impala Tables</title>
  <titlealts audience="PDF"><navtitle>SequenceFile Data Files</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <!-- <data name="Category" value="SequenceFile"/> -->
      <data name="Category" value="File Formats"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p> Impala supports using SequenceFile data files. </p>

    <table>
      <title>SequenceFile Format Support in Impala</title>
      <tgroup cols="5">
        <colspec colname="1" colwidth="10*"/>
        <colspec colname="2" colwidth="10*"/>
        <colspec colname="3" colwidth="20*"/>
        <colspec colname="4" colwidth="30*"/>
        <colspec colname="5" colwidth="30*"/>
        <thead>
          <row>
            <entry>
              File Type
            </entry>
            <entry>
              Format
            </entry>
            <entry>
              Compression Codecs
            </entry>
            <entry>
              Impala Can CREATE?
            </entry>
            <entry>
              Impala Can INSERT?
            </entry>
          </row>
        </thead>
        <tbody>
          <row conref="impala_file_formats.xml#file_formats/sequencefile_support">
            <entry/>
          </row>
        </tbody>
      </tgroup>
    </table>

    <p outputclass="toc inpage"/>
  </conbody>

  <concept id="seqfile_create">

    <title>Creating SequenceFile Tables and Loading Data</title>
  <prolog>
    <metadata>
      <data name="Category" value="ETL"/>
    </metadata>
  </prolog>

    <conbody>

      <p>
        If you do not have an existing data file to use, begin by creating one in the appropriate format.
      </p>

      <p>
        <b>To create a SequenceFile table:</b>
      </p>

      <p>
        In the <codeph>impala-shell</codeph> interpreter, issue a command similar to:
      </p>

<codeblock>create table sequencefile_table (<varname>column_specs</varname>) stored as sequencefile;</codeblock>

      <p>
        Because Impala can query some kinds of tables that it cannot currently write to, after creating tables of
        certain file formats, you might use the Hive shell to load the data. See
        <xref href="impala_file_formats.xml#file_formats"/> for details. After loading data into a table through
        Hive or other mechanism outside of Impala, issue a <codeph>REFRESH <varname>table_name</varname></codeph>
        statement the next time you connect to the Impala node, before querying the table, to make Impala recognize
        the new data.
      </p>

      <p>
        For example, here is how you might create some SequenceFile tables in Impala (by specifying the columns
        explicitly, or cloning the structure of another table), load data through Hive, and query them through
        Impala:
      </p>

<codeblock>$ impala-shell -i localhost
[localhost:21000] &gt; create table seqfile_table (x int) stored as sequencefile;
[localhost:21000] &gt; create table seqfile_clone like some_other_table stored as sequencefile;
[localhost:21000] &gt; quit;

$ hive
hive&gt; insert into table seqfile_table select x from some_other_table;
3 Rows loaded to seqfile_table
Time taken: 19.047 seconds
hive&gt; quit;

$ impala-shell -i localhost
[localhost:21000] &gt; select * from seqfile_table;
Returned 0 row(s) in 0.23s
[localhost:21000] &gt; -- Make Impala recognize the data loaded through Hive;
[localhost:21000] &gt; refresh seqfile_table;
[localhost:21000] &gt; select * from seqfile_table;
+---+
| x |
+---+
| 1 |
| 2 |
| 3 |
+---+
Returned 3 row(s) in 0.23s</codeblock>

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

    </conbody>
  </concept>

  <concept id="seqfile_compression">

    <title>Enabling Compression for SequenceFile Tables</title>
  <prolog>
    <metadata>
      <data name="Category" value="Compression"/>
      <data name="Category" value="Snappy"/>
    </metadata>
  </prolog>

    <conbody>

      <p> You may want to enable compression on existing tables. Enabling
        compression provides performance gains in most cases and is supported
        for SequenceFile tables. For example, to enable Snappy compression, you
        would specify the following additional settings when loading data
        through the Hive shell: </p>

<codeblock>hive&gt; SET hive.exec.compress.output=true;
hive&gt; SET mapred.max.split.size=256000000;
hive&gt; SET mapred.output.compression.type=BLOCK;
hive&gt; SET mapred.output.compression.codec=org.apache.hadoop.io.compress.SnappyCodec;
hive&gt; insert overwrite table <varname>new_table</varname> select * from <varname>old_table</varname>;</codeblock>

      <p>
        If you are converting partitioned tables, you must complete additional steps. In such a case, specify
        additional settings similar to the following:
      </p>

<codeblock>hive&gt; create table <varname>new_table</varname> (<varname>your_cols</varname>) partitioned by (<varname>partition_cols</varname>) stored as <varname>new_format</varname>;
hive&gt; SET hive.exec.dynamic.partition.mode=nonstrict;
hive&gt; SET hive.exec.dynamic.partition=true;
hive&gt; insert overwrite table <varname>new_table</varname> partition(<varname>comma_separated_partition_cols</varname>) select * from <varname>old_table</varname>;</codeblock>

      <p>
        Remember that Hive does not require that you specify a source format for it. Consider the case of
        converting a table with two partition columns called <codeph>year</codeph> and <codeph>month</codeph> to a
        Snappy compressed SequenceFile. Combining the components outlined previously to complete this table
        conversion, you would specify settings similar to the following:
      </p>

<codeblock>hive&gt; create table TBL_SEQ (int_col int, string_col string) STORED AS SEQUENCEFILE;
hive&gt; SET hive.exec.compress.output=true;
hive&gt; SET mapred.max.split.size=256000000;
hive&gt; SET mapred.output.compression.type=BLOCK;
hive&gt; SET mapred.output.compression.codec=org.apache.hadoop.io.compress.SnappyCodec;
hive&gt; SET hive.exec.dynamic.partition.mode=nonstrict;
hive&gt; SET hive.exec.dynamic.partition=true;
hive&gt; INSERT OVERWRITE TABLE tbl_seq SELECT * FROM tbl;</codeblock>

      <p>
        To complete a similar process for a table that includes partitions, you would specify settings similar to
        the following:
      </p>

<codeblock>hive&gt; CREATE TABLE tbl_seq (int_col INT, string_col STRING) PARTITIONED BY (year INT) STORED AS SEQUENCEFILE;
hive&gt; SET hive.exec.compress.output=true;
hive&gt; SET mapred.max.split.size=256000000;
hive&gt; SET mapred.output.compression.type=BLOCK;
hive&gt; SET mapred.output.compression.codec=org.apache.hadoop.io.compress.SnappyCodec;
hive&gt; SET hive.exec.dynamic.partition.mode=nonstrict;
hive&gt; SET hive.exec.dynamic.partition=true;
hive&gt; INSERT OVERWRITE TABLE tbl_seq PARTITION(year) SELECT * FROM tbl;</codeblock>

      <note>
        <p>
          The compression type is specified in the following command:
        </p>
<codeblock>SET mapred.output.compression.codec=org.apache.hadoop.io.compress.SnappyCodec;</codeblock>
        <p>
          You could elect to specify alternative codecs such as <codeph>GzipCodec</codeph> here.
        </p>
      </note>
    </conbody>
  </concept>

  <concept id="seqfile_performance">

    <title>Query Performance for Impala SequenceFile Tables</title>

    <conbody>

      <p>
        In general, expect query performance with SequenceFile tables to be
        faster than with tables using text data, but slower than with
        Parquet tables. See <xref href="impala_parquet.xml#parquet"/>
        for information about using the Parquet file format for
        high-performance analytic queries.
      </p>

      <p conref="../shared/impala_common.xml#common/s3_block_splitting"/>

    </conbody>
  </concept>

</concept>