impala/docs/topics/impala_langref_unsupported.xml

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

  <title>SQL Differences Between Impala and Hive</title>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Hive"/>
      <data name="Category" value="Porting"/>
      <data name="Category" value="Data Analysts"/>
      <data name="Category" value="Developers"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">Hive</indexterm>
      <indexterm audience="hidden">HiveQL</indexterm>
      Impala's SQL syntax follows the SQL-92 standard, and includes many industry extensions in areas such as
      built-in functions. See <xref href="impala_porting.xml#porting"/> for a general discussion of adapting SQL
      code from a variety of database systems to Impala.
    </p>

    <p>
      Because Impala and Hive share the same metastore database and their tables are often used interchangeably,
      the following section covers differences between Impala and Hive in detail.
    </p>

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

  <concept id="langref_hiveql_unsupported">

    <title>HiveQL Features not Available in Impala</title>

    <conbody>

      <p>
        The current release of Impala does not support the following SQL features that you might be familiar with
        from HiveQL:
      </p>

      <!-- To do:
        Yeesh, too many separate lists of unsupported Hive syntax.
        Here, the FAQ, and in some of the intro topics.
        Some discussion in IMP-1061 about how best to reorg.
        Lots of opportunities for conrefs.
      -->

      <ul>
<!-- Now supported in <keyword keyref="impala23_full"/> and higher. Find places on this page (like already done under lateral views) to note the new data type support.
        <li>
          Non-scalar data types such as maps, arrays, structs.
        </li>
-->

        <li rev="1.2">
          Extensibility mechanisms such as <codeph>TRANSFORM</codeph>, custom file formats, or custom SerDes.
        </li>

        <li rev="">
          The <codeph>DATE</codeph> data type.
        </li>

        <li>
          XML and JSON functions.
        </li>

        <li>
          Certain aggregate functions from HiveQL: <codeph>covar_pop</codeph>, <codeph>covar_samp</codeph>,
          <codeph>corr</codeph>, <codeph>percentile</codeph>, <codeph>percentile_approx</codeph>,
          <codeph>histogram_numeric</codeph>, <codeph>collect_set</codeph>; Impala supports the set of aggregate
          functions listed in <xref href="impala_aggregate_functions.xml#aggregate_functions"/> and analytic
          functions listed in <xref href="impala_analytic_functions.xml#analytic_functions"/>.
        </li>

        <li>
          Sampling.
        </li>

        <li>
          Lateral views. In <keyword keyref="impala23_full"/> and higher, Impala supports queries on complex types
          (<codeph>STRUCT</codeph>, <codeph>ARRAY</codeph>, or <codeph>MAP</codeph>), using join notation
          rather than the <codeph>EXPLODE()</codeph> keyword.
          See <xref href="impala_complex_types.xml#complex_types"/> for details about Impala support for complex types.
        </li>

        <li>
          Multiple <codeph>DISTINCT</codeph> clauses per query, although Impala includes some workarounds for this
          limitation.
          <note conref="../shared/impala_common.xml#common/multiple_count_distinct"/>
        </li>
      </ul>

      <p>
        User-defined functions (UDFs) are supported starting in Impala 1.2. See <xref href="impala_udf.xml#udfs"/>
        for full details on Impala UDFs.
        <ul>
          <li>
            <p>
              Impala supports high-performance UDFs written in C++, as well as reusing some Java-based Hive UDFs.
            </p>
          </li>

          <li>
            <p>
              Impala supports scalar UDFs and user-defined aggregate functions (UDAFs). Impala does not currently
              support user-defined table generating functions (UDTFs).
            </p>
          </li>

          <li>
            <p>
              Only Impala-supported column types are supported in Java-based UDFs.
            </p>
          </li>

          <li>
            <p conref="../shared/impala_common.xml#common/current_user_caveat"/>
          </li>
        </ul>
      </p>

      <p>
        Impala does not currently support these HiveQL statements:
      </p>

      <ul>
        <li>
          <codeph>ANALYZE TABLE</codeph> (the Impala equivalent is <codeph>COMPUTE STATS</codeph>)
        </li>

        <li>
          <codeph>DESCRIBE COLUMN</codeph>
        </li>

        <li>
          <codeph>DESCRIBE DATABASE</codeph>
        </li>

        <li>
          <codeph>EXPORT TABLE</codeph>
        </li>

        <li>
          <codeph>IMPORT TABLE</codeph>
        </li>

        <li>
          <codeph>SHOW TABLE EXTENDED</codeph>
        </li>

        <li>
          <codeph>SHOW INDEXES</codeph>
        </li>

        <li>
          <codeph>SHOW COLUMNS</codeph>
        </li>

        <li rev="DOCS-656">
          <codeph>INSERT OVERWRITE DIRECTORY</codeph>; use <codeph>INSERT OVERWRITE <varname>table_name</varname></codeph>
          or <codeph>CREATE TABLE AS SELECT</codeph> to materialize query results into the HDFS directory associated
          with an Impala table.
        </li>
      </ul>
    </conbody>
  </concept>

  <concept id="langref_hiveql_semantics">

    <title>Semantic Differences Between Impala and HiveQL Features</title>

    <conbody>

      <p>
        This section covers instances where Impala and Hive have similar functionality, sometimes including the
        same syntax, but there are differences in the runtime semantics of those features.
      </p>

      <p>
        <b>Security:</b>
      </p>

      <p>
        Impala utilizes the <xref href="http://sentry.incubator.apache.org/" scope="external" format="html">Apache
        Sentry </xref> authorization framework, which provides fine-grained role-based access control
        to protect data against unauthorized access or tampering.
      </p>

      <p>
        The Hive component now includes Sentry-enabled <codeph>GRANT</codeph>,
        <codeph>REVOKE</codeph>, and <codeph>CREATE/DROP ROLE</codeph> statements. Earlier Hive releases had a
        privilege system with <codeph>GRANT</codeph> and <codeph>REVOKE</codeph> statements that were primarily
        intended to prevent accidental deletion of data, rather than a security mechanism to protect against
        malicious users.
      </p>

      <p>
        Impala can make use of privileges set up through Hive <codeph>GRANT</codeph> and <codeph>REVOKE</codeph> statements.
        Impala has its own <codeph>GRANT</codeph> and <codeph>REVOKE</codeph> statements in Impala 2.0 and higher.
        See <xref href="impala_authorization.xml#authorization"/> for the details of authorization in Impala, including
        how to switch from the original policy file-based privilege model to the Sentry service using privileges
        stored in the metastore database.
      </p>

      <p>
        <b>SQL statements and clauses:</b>
      </p>

      <p>
        The semantics of Impala SQL statements varies from HiveQL in some cases where they use similar SQL
        statement and clause names:
      </p>

      <ul>
        <li>
          Impala uses different syntax and names for query hints, <codeph>[SHUFFLE]</codeph> and
          <codeph>[NOSHUFFLE]</codeph> rather than <codeph>MapJoin</codeph> or <codeph>StreamJoin</codeph>. See
          <xref href="impala_joins.xml#joins"/> for the Impala details.
        </li>

        <li>
          Impala does not expose MapReduce specific features of <codeph>SORT BY</codeph>, <codeph>DISTRIBUTE
          BY</codeph>, or <codeph>CLUSTER BY</codeph>.
        </li>

        <li>
          Impala does not require queries to include a <codeph>FROM</codeph> clause.
        </li>
      </ul>

      <p>
        <b>Data types:</b>
      </p>

      <ul>
        <li>
          Impala supports a limited set of implicit casts. This can help avoid undesired results from unexpected
          casting behavior.
          <ul>
            <li>
              Impala does not implicitly cast between string and numeric or Boolean types. Always use
              <codeph>CAST()</codeph> for these conversions.
            </li>

            <li>
              Impala does perform implicit casts among the numeric types, when going from a smaller or less precise
              type to a larger or more precise one. For example, Impala will implicitly convert a
              <codeph>SMALLINT</codeph> to a <codeph>BIGINT</codeph> or <codeph>FLOAT</codeph>, but to convert from
              <codeph>DOUBLE</codeph> to <codeph>FLOAT</codeph> or <codeph>INT</codeph> to <codeph>TINYINT</codeph>
              requires a call to <codeph>CAST()</codeph> in the query.
            </li>

            <li>
              Impala does perform implicit casts from string to timestamp. Impala has a restricted set of literal
              formats for the <codeph>TIMESTAMP</codeph> data type and the <codeph>from_unixtime()</codeph> format
              string; see <xref href="impala_timestamp.xml#timestamp"/> for details.
            </li>
          </ul>
          <p>
            See <xref href="impala_datatypes.xml#datatypes"/> for full details on implicit and explicit casting for
            all types, and <xref href="impala_conversion_functions.xml#conversion_functions"/> for details about
            the <codeph>CAST()</codeph> function.
          </p>
        </li>

        <li>
          Impala does not store or interpret timestamps using the local timezone, to avoid undesired results from
          unexpected time zone issues. Timestamps are stored and interpreted relative to UTC. This difference can
          produce different results for some calls to similarly named date/time functions between Impala and Hive.
          See <xref href="impala_datetime_functions.xml#datetime_functions"/> for details about the Impala
          functions. See <xref href="impala_timestamp.xml#timestamp"/> for a discussion of how Impala handles
          time zones, and configuration options you can use to make Impala match the Hive behavior more closely
          when dealing with Parquet-encoded <codeph>TIMESTAMP</codeph> data or when converting between
          the local time zone and UTC.
        </li>

        <li>
          The Impala <codeph>TIMESTAMP</codeph> type can represent dates ranging from 1400-01-01 to 9999-12-31.
          This is different from the Hive date range, which is 0000-01-01 to 9999-12-31.
        </li>

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

      </ul>

      <p>
        <b>Miscellaneous features:</b>
      </p>

      <ul>
        <li>
          Impala does not provide virtual columns.
        </li>

        <li>
          Impala does not expose locking.
        </li>

        <li>
          Impala does not expose some configuration properties.
        </li>
      </ul>
    </conbody>
  </concept>
</concept>