impala-commits mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From jbap...@apache.org
Subject [02/51] [partial] incubator-impala git commit: IMPALA-4181 [DOCS] Publish rendered Impala documentation to ASF site
Date Wed, 12 Apr 2017 18:25:06 GMT
http://git-wip-us.apache.org/repos/asf/incubator-impala/blob/75c46918/docs/build/html/topics/impala_troubleshooting.html
----------------------------------------------------------------------
diff --git a/docs/build/html/topics/impala_troubleshooting.html b/docs/build/html/topics/impala_troubleshooting.html
new file mode 100644
index 0000000..7728ee4
--- /dev/null
+++ b/docs/build/html/topics/impala_troubleshooting.html
@@ -0,0 +1,370 @@
+<!DOCTYPE html
+  SYSTEM "about:legacy-compat">
+<html lang="en"><head><meta http-equiv="Content-Type" content="text/html;
charset=UTF-8"><meta charset="UTF-8"><meta name="copyright" content="(C) Copyright
2017"><meta name="DC.rights.owner" content="(C) Copyright 2017"><meta name="DC.Type"
content="concept"><meta name="DC.Relation" scheme="URI" content="../topics/impala_webui.html"><meta
name="DC.Relation" scheme="URI" content="../topics/impala_breakpad.html"><meta name="prodname"
content="Impala"><meta name="prodname" content="Impala"><meta name="version" content="Impala
2.8.x"><meta name="version" content="Impala 2.8.x"><meta name="DC.Format" content="XHTML"><meta
name="DC.Identifier" content="troubleshooting"><link rel="stylesheet" type="text/css"
href="../commonltr.css"><title>Troubleshooting Impala</title></head><body
id="troubleshooting"><main role="main"><article role="article" aria-labelledby="ariaid-title1">
+
+  <h1 class="title topictitle1" id="ariaid-title1">Troubleshooting Impala</h1>
+  
+
+  <div class="body conbody">
+
+    <p class="p">
+      
+      Troubleshooting for Impala requires being able to diagnose and debug problems
+      with performance, network connectivity, out-of-memory conditions, disk space usage,
+      and crash or hang conditions in any of the Impala-related daemons.
+    </p>
+
+    <p class="p toc inpage">
+      The following sections describe the general troubleshooting procedures to diagnose
+      different kinds of problems:
+    </p>
+
+  </div>
+
+  <nav role="navigation" class="related-links"><ul class="ullinks"><li class="link
ulchildlink"><strong><a href="../topics/impala_webui.html">Impala Web User
Interface for Debugging</a></strong><br></li><li class="link ulchildlink"><strong><a
href="../topics/impala_breakpad.html">Breakpad Minidumps for Impala (Impala 2.6 or higher
only)</a></strong><br></li></ul></nav><article class="topic
concept nested1" aria-labelledby="ariaid-title2" id="troubleshooting__trouble_sql">
+
+    <h2 class="title topictitle2" id="ariaid-title2">Troubleshooting Impala SQL Syntax
Issues</h2>
+
+    <div class="body conbody">
+
+      <p class="p">
+        In general, if queries issued against Impala fail, you can try running these same
queries against Hive.
+      </p>
+
+      <ul class="ul">
+        <li class="li">
+          If a query fails against both Impala and Hive, it is likely that there is a problem
with your query or
+          other elements of your <span class="keyword"></span> environment:
+          <ul class="ul">
+            <li class="li">
+              Review the <a class="xref" href="impala_langref.html#langref">Language
Reference</a> to ensure your query is
+              valid.
+            </li>
+
+            <li class="li">
+              Check <a class="xref" href="impala_reserved_words.html#reserved_words">Impala
Reserved Words</a> to see if any database, table,
+              column, or other object names in your query conflict with Impala reserved words.
+              Quote those names with backticks (<code class="ph codeph">``</code>)
if so.
+            </li>
+
+            <li class="li">
+              Check <a class="xref" href="impala_functions.html#builtins">Impala Built-In
Functions</a> to confirm whether Impala supports all the
+              built-in functions being used by your query, and whether argument and return
types are the
+              same as you expect.
+            </li>
+
+            <li class="li">
+              Review the <a class="xref" href="impala_logging.html#logs_debug">contents
of the Impala logs</a> for any information that may be useful in identifying the
+              source of the problem.
+            </li>
+          </ul>
+        </li>
+
+        <li class="li">
+          If a query fails against Impala but not Hive, it is likely that there is a problem
with your Impala
+          installation.
+        </li>
+      </ul>
+    </div>
+  </article>
+
+  <article class="topic concept nested1" aria-labelledby="ariaid-title3" id="troubleshooting__trouble_io">
+    <h2 class="title topictitle2" id="ariaid-title3">Troubleshooting I/O Capacity Problems</h2>
+    <div class="body conbody">
+      <p class="p">
+        Impala queries are typically I/O-intensive. If there is an I/O problem with storage
devices,
+        or with HDFS itself, Impala queries could show slow response times with no obvious
cause
+        on the Impala side. Slow I/O on even a single DataNode could result in an overall
slowdown, because
+        queries involving clauses such as <code class="ph codeph">ORDER BY</code>,
<code class="ph codeph">GROUP BY</code>, or <code class="ph codeph">JOIN</code>
+        do not start returning results until all DataNodes have finished their work.
+      </p>
+      <p class="p">
+        To test whether the Linux I/O system itself is performing as expected, run Linux
commands like
+        the following on each DataNode:
+      </p>
+<pre class="pre codeblock"><code>
+$ sudo sysctl -w vm.drop_caches=3 vm.drop_caches=0
+vm.drop_caches = 3
+vm.drop_caches = 0
+$ sudo dd if=/dev/sda bs=1M of=/dev/null count=1k 
+1024+0 records in
+1024+0 records out
+1073741824 bytes (1.1 GB) copied, 5.60373 s, 192 MB/s
+$ sudo dd if=/dev/sdb bs=1M of=/dev/null count=1k
+1024+0 records in
+1024+0 records out
+1073741824 bytes (1.1 GB) copied, 5.51145 s, 195 MB/s
+$ sudo dd if=/dev/sdc bs=1M of=/dev/null count=1k
+1024+0 records in
+1024+0 records out
+1073741824 bytes (1.1 GB) copied, 5.58096 s, 192 MB/s
+$ sudo dd if=/dev/sdd bs=1M of=/dev/null count=1k
+1024+0 records in
+1024+0 records out
+1073741824 bytes (1.1 GB) copied, 5.43924 s, 197 MB/s
+</code></pre>
+      <p class="p">
+        On modern hardware, a throughput rate of less than 100 MB/s typically indicates
+        a performance issue with the storage device. Correct the hardware problem before
+        continuing with Impala tuning or benchmarking.
+      </p>
+    </div>
+  </article>
+
+
+  <article class="topic concept nested1" aria-labelledby="ariaid-title4" id="troubleshooting__trouble_cookbook">
+
+    <h2 class="title topictitle2" id="ariaid-title4">Impala Troubleshooting Quick Reference</h2>
+
+    <div class="body conbody">
+
+      <p class="p">
+        The following table lists common problems and potential solutions.
+      </p>
+
+      <table class="table"><caption></caption><colgroup><col style="width:14.285714285714285%"><col
style="width:42.857142857142854%"><col style="width:42.857142857142854%"></colgroup><thead
class="thead">
+            <tr class="row">
+              <th class="entry nocellnorowborder" id="trouble_cookbook__entry__1">
+                Symptom
+              </th>
+              <th class="entry nocellnorowborder" id="trouble_cookbook__entry__2">
+                Explanation
+              </th>
+              <th class="entry nocellnorowborder" id="trouble_cookbook__entry__3">
+                Recommendation
+              </th>
+            </tr>
+          </thead><tbody class="tbody">
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                Impala takes a long time to start.
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                Impala instances with large numbers of tables, partitions, or data files
take longer to start
+                because the metadata for these objects is broadcast to all <span class="keyword
cmdname">impalad</span> nodes and
+                cached.
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                Adjust timeout and synchronicity settings.
+              </td>
+            </tr>
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                <p class="p">
+                  Joins fail to complete.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                <p class="p">
+                  There may be insufficient memory. During a join, data from the second,
third, and so on sets to
+                  be joined is loaded into memory. If Impala chooses an inefficient join
order or join mechanism,
+                  the query could exceed the total memory available.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                <p class="p">
+                  Start by gathering statistics with the <code class="ph codeph">COMPUTE
STATS</code> statement for each table
+                  involved in the join. Consider specifying the <code class="ph codeph">[SHUFFLE]</code>
hint so that data from
+                  the joined tables is split up between nodes rather than broadcast to each
node. If tuning at the
+                  SQL level is not sufficient, add more memory to your system or join smaller
data sets.
+                </p>
+              </td>
+            </tr>
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                <p class="p">
+                  Queries return incorrect results.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                <p class="p">
+                  Impala metadata may be outdated after changes are performed in Hive.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                <p class="p">
+                  Where possible, use the appropriate Impala statement (<code class="ph
codeph">INSERT</code>, <code class="ph codeph">LOAD
+                  DATA</code>, <code class="ph codeph">CREATE TABLE</code>,
<code class="ph codeph">ALTER TABLE</code>, <code class="ph codeph">COMPUTE
+                  STATS</code>, and so on) rather than switching back and forth between
Impala and Hive. Impala
+                  automatically broadcasts the results of DDL and DML operations to all Impala
nodes in the
+                  cluster, but does not automatically recognize when such changes are made
through Hive. After
+                  inserting data, adding a partition, or other operation in Hive, refresh
the metadata for the
+                  table as described in <a class="xref" href="impala_refresh.html#refresh">REFRESH
Statement</a>.
+                </p>
+              </td>
+            </tr>
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                <p class="p">
+                  Queries are slow to return results.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                <p class="p">
+                  Some <code class="ph codeph">impalad</code> instances may not
have started. Using a browser, connect to the
+                  host running the Impala state store. Connect using an address of the form
+                  <code class="ph codeph">http://<var class="keyword varname">hostname</var>:<var
class="keyword varname">port</var>/metrics</code>.
+                </p>
+
+                <div class="p">
+                  <div class="note note note_note"><span class="note__title notetitle">Note:</span>

+                    Replace <var class="keyword varname">hostname</var> and <var
class="keyword varname">port</var> with the hostname and port of
+                    your Impala state store host machine and web server port. The default
port is 25010.
+                  </div>
+                  The number of <code class="ph codeph">impalad</code> instances
listed should match the expected number of
+                  <code class="ph codeph">impalad</code> instances installed
in the cluster. There should also be one
+                  <code class="ph codeph">impalad</code> instance installed on
each DataNode
+                </div>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                <p class="p">
+                  Ensure Impala is installed on all DataNodes. Start any <code class="ph
codeph">impalad</code> instances that
+                  are not running.
+                </p>
+              </td>
+            </tr>
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                <p class="p">
+                  Queries are slow to return results.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                <p class="p">
+                  Impala may not be configured to use native checksumming. Native checksumming
uses
+                  machine-specific instructions to compute checksums over HDFS data very
quickly. Review Impala
+                  logs. If you find instances of "<code class="ph codeph">INFO util.NativeCodeLoader:
Loaded the
+                  native-hadoop</code>" messages, native checksumming is not enabled.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                <p class="p">
+                  Ensure Impala is configured to use native checksumming as described in
+                  <a class="xref" href="impala_config_performance.html#config_performance">Post-Installation
Configuration for Impala</a>.
+                </p>
+              </td>
+            </tr>
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                <p class="p">
+                  Queries are slow to return results.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                <p class="p">
+                  Impala may not be configured to use data locality tracking.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                <p class="p">
+                  Test Impala for data locality tracking and make configuration changes as
necessary. Information
+                  on this process can be found in <a class="xref" href="impala_config_performance.html#config_performance">Post-Installation
Configuration for Impala</a>.
+                </p>
+              </td>
+            </tr>
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                <p class="p">
+                  Attempts to complete Impala tasks such as executing INSERT-SELECT actions
fail. The Impala logs
+                  include notes that files could not be opened due to permission denied.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                <p class="p">
+                  This can be the result of permissions issues. For example, you could use
the Hive shell as the
+                  hive user to create a table. After creating this table, you could attempt
to complete some
+                  action, such as an INSERT-SELECT on the table. Because the table was created
using one user and
+                  the INSERT-SELECT is attempted by another, this action may fail due to
permissions issues.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                <p class="p">
+                  In general, ensure the Impala user has sufficient permissions. In the preceding
example, ensure
+                  the Impala user has sufficient permissions to the table that the Hive user
created.
+                </p>
+              </td>
+            </tr>
+            <tr class="row">
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__1
">
+                <p class="p">
+                  Impala fails to start up, with the <span class="keyword cmdname">impalad</span>
logs referring to errors connecting
+                  to the statestore service and attempts to re-register.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__2
">
+                <p class="p">
+                  A large number of databases, tables, partitions, and so on can require
metadata synchronization,
+                  particularly on startup, that takes longer than the default timeout for
the statestore service.
+                </p>
+              </td>
+              <td class="entry nocellnorowborder" headers="trouble_cookbook__entry__3
">
+                <p class="p">
+                  Configure the statestore timeout value and possibly other settings related
to the frequency of
+                  statestore updates and metadata loading. See
+                  <a class="xref" href="impala_timeouts.html#statestore_timeout">Increasing
the Statestore Timeout</a> and
+                  <a class="xref" href="impala_scalability.html#statestore_scalability">Scalability
Considerations for the Impala Statestore</a>.
+                </p>
+              </td>
+            </tr>
+          </tbody></table>
+
+      
+    </div>
+  </article>
+
+  
+
+  
+
+  <article class="topic concept nested1" aria-labelledby="ariaid-title5" id="troubleshooting__webui_snippet">
+    <h2 class="title topictitle2" id="ariaid-title5">Impala Web User Interface for
Debugging</h2>
+    <div class="body conbody">
+      <div class="p">
+      
+      
+      Each of the Impala daemons (<span class="keyword cmdname">impalad</span>,
<span class="keyword cmdname">statestored</span>,
+      and <span class="keyword cmdname">catalogd</span>) includes a built-in
web server that displays
+      diagnostic and status information:
+      <ul class="ul">
+      <li class="li">
+        <p class="p">
+          The <span class="keyword cmdname">impalad</span> web UI (default port:
25000) includes
+          information about configuration settings, running and completed queries, and associated
performance and
+          resource usage for queries. In particular, the <span class="ph uicontrol">Details</span>
link for each query displays
+          alternative views of the query including a graphical representation of the plan,
and the
+          output of the <code class="ph codeph">EXPLAIN</code>, <code class="ph
codeph">SUMMARY</code>, and <code class="ph codeph">PROFILE</code>
+          statements from <span class="keyword cmdname">impala-shell</span>.
+          Each host that runs the <span class="keyword cmdname">impalad</span>
daemon has
+          its own instance of the web UI, with details about those queries for which that
+          host served as the coordinator. The <span class="keyword cmdname">impalad</span>
web UI is mainly
+          for diagnosing query problems that can be traced to a particular node.
+        </p>
+      </li>
+      <li class="li">
+        <p class="p">
+          The <span class="keyword cmdname">statestored</span> web UI (default
port: 25010) includes
+          information about memory usage, configuration settings, and ongoing health checks
+          performed by this daemon. Because there is only a single instance of this
+          daemon within any cluster, you view the web UI only on the particular host
+          that serves as the Impala Statestore.
+        </p>
+      </li>
+      <li class="li">
+        <p class="p">
+          The <span class="keyword cmdname">catalogd</span> web UI (default port:
25020) includes
+          information about the databases, tables, and other objects managed by Impala,
+          in addition to the resource usage and configuration settings of the daemon itself.
+          The catalog information is represented as the underlying Thrift data structures.
+          Because there is only a single instance of this daemon within any cluster, you
view the
+          web UI only on the particular host that serves as the Impala Catalog Server.
+        </p>
+      </li>
+      </ul>
+    </div>
+      <p class="p">
+        For full details, see <a class="xref" href="impala_webui.html#webui">Impala
Web User Interface for Debugging</a>.
+      </p>
+    </div>
+  </article>
+
+</article></main></body></html>
\ No newline at end of file

http://git-wip-us.apache.org/repos/asf/incubator-impala/blob/75c46918/docs/build/html/topics/impala_truncate_table.html
----------------------------------------------------------------------
diff --git a/docs/build/html/topics/impala_truncate_table.html b/docs/build/html/topics/impala_truncate_table.html
new file mode 100644
index 0000000..9e5b530
--- /dev/null
+++ b/docs/build/html/topics/impala_truncate_table.html
@@ -0,0 +1,200 @@
+<!DOCTYPE html
+  SYSTEM "about:legacy-compat">
+<html lang="en"><head><meta http-equiv="Content-Type" content="text/html;
charset=UTF-8"><meta charset="UTF-8"><meta name="copyright" content="(C) Copyright
2017"><meta name="DC.rights.owner" content="(C) Copyright 2017"><meta name="DC.Type"
content="concept"><meta name="DC.Relation" scheme="URI" content="../topics/impala_langref_sql.html"><meta
name="prodname" content="Impala"><meta name="prodname" content="Impala"><meta
name="version" content="Impala 2.8.x"><meta name="version" content="Impala 2.8.x"><meta
name="DC.Format" content="XHTML"><meta name="DC.Identifier" content="truncate_table"><link
rel="stylesheet" type="text/css" href="../commonltr.css"><title>TRUNCATE TABLE Statement
(Impala 2.3 or higher only)</title></head><body id="truncate_table"><main
role="main"><article role="article" aria-labelledby="ariaid-title1">
+
+  <h1 class="title topictitle1" id="ariaid-title1">TRUNCATE TABLE Statement (<span
class="keyword">Impala 2.3</span> or higher only)</h1>
+  
+  
+
+  <div class="body conbody">
+
+    <p class="p">
+      
+      Removes the data from an Impala table while leaving the table itself.
+    </p>
+
+    <p class="p">
+        <strong class="ph b">Syntax:</strong>
+      </p>
+
+<pre class="pre codeblock"><code>TRUNCATE TABLE <span class="ph">[IF EXISTS]</span>
[<var class="keyword varname">db_name</var>.]<var class="keyword varname">table_name</var></code></pre>
+
+    <p class="p">
+        <strong class="ph b">Statement type:</strong> DDL
+      </p>
+
+    <p class="p">
+        <strong class="ph b">Usage notes:</strong>
+      </p>
+
+    <p class="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 <code class="ph codeph">INSERT OVERWRITE</code>
to replace the data during the
+      next ETL cycle.
+    </p>
+
+    <p class="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 class="p">
+      Any statistics produced by the <code class="ph codeph">COMPUTE STATS</code>
statement are reset when the data is removed.
+    </p>
+
+    <p class="p">
+      Make sure that you are in the correct database before truncating a table, either by
issuing a
+      <code class="ph codeph">USE</code> statement first or by using a fully
qualified name
+      <code class="ph codeph"><var class="keyword varname">db_name</var>.<var
class="keyword varname">table_name</var></code>.
+    </p>
+
+    <p class="p">
+      The optional <code class="ph codeph">IF EXISTS</code> 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 <span class="keyword">Impala 2.5</span> and
higher.
+    </p>
+
+    <p class="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 class="p">
+        For other tips about managing and reclaiming Impala disk space, see
+        <a class="xref" href="../shared/../topics/impala_disk_space.html#disk_space">Managing
Disk Space for Impala Data</a>.
+      </p>
+
+    <p class="p">
+        <strong class="ph b">Amazon S3 considerations:</strong>
+      </p>
+    <p class="p">
+      Although Impala cannot write new data to a table stored in the Amazon
+      S3 filesystem, the <code class="ph codeph">TRUNCATE TABLE</code> statement
can remove data files from S3.
+      See <a class="xref" href="impala_s3.html#s3">Using Impala with the Amazon S3
Filesystem</a> for details about working with S3 tables.
+    </p>
+
+    <p class="p">
+        <strong class="ph b">Cancellation:</strong> Cannot be cancelled.
+      </p>
+
+    <p class="p">
+        <strong class="ph b">HDFS permissions:</strong>
+      </p>
+    <p class="p">
+      The user ID that the <span class="keyword cmdname">impalad</span> daemon
runs under,
+      typically the <code class="ph codeph">impala</code> user, must have write
+      permission for all the files and directories that make up the table.
+    </p>
+
+    <p class="p">
+        <strong class="ph b">Kudu considerations:</strong>
+      </p>
+    <p class="p">
+        Currently, the <code class="ph codeph">TRUNCATE TABLE</code> statement
cannot be used with Kudu tables.
+      </p>
+
+    <p class="p">
+        <strong class="ph b">Examples:</strong>
+      </p>
+
+    <p class="p">
+      The following example shows a table containing some data and with table and column
statistics.
+      After the <code class="ph codeph">TRUNCATE TABLE</code> statement, the
data is removed and the statistics
+      are reset.
+    </p>
+
+<pre class="pre codeblock"><code>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        |
++--------+------+------------------+--------+----------+----------+
+</code></pre>
+
+    <p class="p">
+      The following example shows how the <code class="ph codeph">IF EXISTS</code>
clause allows the <code class="ph codeph">TRUNCATE TABLE</code>
+      statement to be run without error whether or not the table exists:
+    </p>
+
+<pre class="pre codeblock"><code>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
+</code></pre>
+
+    <p class="p">
+        <strong class="ph b">Related information:</strong>
+      </p>
+
+    <p class="p">
+      <a class="xref" href="impala_tables.html#tables">Overview of Impala Tables</a>,
+      <a class="xref" href="impala_alter_table.html#alter_table">ALTER TABLE Statement</a>,
<a class="xref" href="impala_create_table.html#create_table">CREATE TABLE Statement</a>,
+      <a class="xref" href="impala_partitioning.html#partitioning">Partitioning for
Impala Tables</a>, <a class="xref" href="impala_tables.html#internal_tables">Internal
Tables</a>,
+      <a class="xref" href="impala_tables.html#external_tables">External Tables</a>
+    </p>
+
+  </div>
+<nav role="navigation" class="related-links"><div class="familylinks"><div
class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_langref_sql.html">Impala
SQL Statements</a></div></div></nav></article></main></body></html>
\ No newline at end of file


Mime
View raw message