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Lu, Huayi e6fa411af0 DWS DEV 830.201 version 
Reviewed-by: Pruthi, Vineet <vineet.pruthi@t-systems.com>
Co-authored-by: Lu, Huayi <luhuayi@huawei.com>
Co-committed-by: Lu, Huayi <luhuayi@huawei.com>
2024-05-16 07:24:04 +00:00

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<a name="EN-US_TOPIC_0000001188323704"></a><a name="EN-US_TOPIC_0000001188323704"></a>
<h1 class="topictitle1">Creating a Foreign Table</h1>
<div id="body0000001163678223"><p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p12167448953">After operations in <a href="dws_04_0162.html">Creating a Foreign Server</a> are complete, create an HDFS write-only foreign table in the <span id="EN-US_TOPIC_0000001188323704__ph8207163711287">GaussDB(DWS)</span> database to access data stored in HDFS. The foreign table is write-only and can be used only for data export.</p>
<div class="p" id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p33659853185314">The syntax for creating a foreign table is as follows. For details, see <strong id="EN-US_TOPIC_0000001188323704__b1331054085713">CREATE FOREIGN TABLE (SQL on Hadoop or OBS)</strong>.<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_screen34509377483"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
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<span class="normal">11</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">CREATE</span><span class="w"> </span><span class="k">FOREIGN</span><span class="w"> </span><span class="k">TABLE</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="k">IF</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">EXISTS</span><span class="w"> </span><span class="p">]</span><span class="w"> </span><span class="k">table_name</span><span class="w"> </span>
<span class="p">(</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="err">{</span><span class="w"> </span><span class="k">column_name</span><span class="w"> </span><span class="n">type_name</span><span class="w"> </span>
<span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="err">{</span><span class="w"> </span><span class="p">[</span><span class="k">CONSTRAINT</span><span class="w"> </span><span class="k">constraint_name</span><span class="p">]</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="o">|</span>
<span class="w"> </span><span class="p">[</span><span class="k">CONSTRAINT</span><span class="w"> </span><span class="k">constraint_name</span><span class="p">]</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="o">|</span>
<span class="w"> </span><span class="n">column_constraint</span><span class="w"> </span><span class="p">[...]</span><span class="err">}</span><span class="w"> </span><span class="p">]</span><span class="w"> </span><span class="o">|</span>
<span class="w"> </span><span class="n">table_constraint</span><span class="w"> </span><span class="p">[,</span><span class="w"> </span><span class="p">...]</span><span class="err">}</span><span class="w"> </span><span class="p">[,</span><span class="w"> </span><span class="p">...]</span><span class="w"> </span><span class="p">]</span><span class="w"> </span><span class="p">)</span><span class="w"> </span>
<span class="w"> </span><span class="n">SERVER</span><span class="w"> </span><span class="n">dfs_server</span><span class="w"> </span>
<span class="w"> </span><span class="k">OPTIONS</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="err">{</span><span class="w"> </span><span class="n">option_name</span><span class="w"> </span><span class="s1">' value '</span><span class="w"> </span><span class="err">}</span><span class="w"> </span><span class="p">[,</span><span class="w"> </span><span class="p">...]</span><span class="w"> </span><span class="p">)</span><span class="w"> </span>
<span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="err">{</span><span class="k">WRITE</span><span class="w"> </span><span class="k">ONLY</span><span class="w"> </span><span class="err">}</span><span class="p">]</span>
<span class="w"> </span><span class="n">DISTRIBUTE</span><span class="w"> </span><span class="k">BY</span><span class="w"> </span><span class="err">{</span><span class="n">ROUNDROBIN</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="n">REPLICATION</span><span class="err">}</span>
<span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="n">PARTITION</span><span class="w"> </span><span class="k">BY</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="k">column_name</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="p">[</span><span class="w"> </span><span class="n">AUTOMAPPED</span><span class="w"> </span><span class="p">]</span><span class="w"> </span><span class="p">]</span><span class="w"> </span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
</div>
<p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p174842037174819">For example, when creating a foreign table <em id="EN-US_TOPIC_0000001188323704__i14562338634">product_info_ext_obs</em>, configure the parameters in the syntax as follows.</p>
<ul id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_ul645584873919"><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li0456164816391"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b19458104873912">table_name</strong><p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p14459164811399">Specifies the name of the foreign table.</p>
</li><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li25911413571"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b691318717383">Table column definitions</strong><ul id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_ul496273711582"><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li1446294810396"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b846619483394">column_name</strong>: specifies the name of a column in the foreign table.</li><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li2471194833916"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b447510488398">type_name</strong>: specifies the data type of the column.</li></ul>
<p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p64441413214">Multiple columns are separate by commas (,).</p>
</li><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li4488164811399"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b1449034813915">SERVER dfs_server</strong><p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p114921548203913">Specifies the foreign server name of the foreign table. This server must exist. The foreign table connects to OBS/HDFS to read data through the foreign server.</p>
<p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p1192123102315">Enter the name of the foreign server created in <a href="dws_04_0162.html">Creating a Foreign Server</a>.</p>
</li><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li184991948153914"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b450214811394">OPTIONS parameters</strong><p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p05031548113911">These parameters are associated with the foreign table. The key parameters are as follows:</p>
<ul id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_ul16505124883912"><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li86062718555"><strong id="EN-US_TOPIC_0000001188323704__b9108357614519">format</strong>: specifies the format of the exported data file. The ORC format is supported.</li><li id="EN-US_TOPIC_0000001188323704__li976404793314"><strong id="EN-US_TOPIC_0000001188323704__b54891155155215">foldername</strong>: specifies the directory of the data source file in the foreign table, that is, the corresponding file directory in HDFS. This parameter is mandatory for write-only foreign tables and optional for read-only foreign tables.</li><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li55231748173915"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b115261548143916">encoding</strong>: specifies the encoding format of the data source file in the foreign table. The default value is <strong id="EN-US_TOPIC_0000001188323704__b714682716">utf8</strong>.</li><li id="EN-US_TOPIC_0000001188323704__li1241716511227"><strong id="EN-US_TOPIC_0000001188323704__b375294532516">filesize</strong><p id="EN-US_TOPIC_0000001188323704__p1141934762310">(Optional) Specifies the file size of a write-only foreign table. If this parameter is not specified, the file size in the distributed file system is used by default. This syntax is available only for the write-only foreign table.</p>
<p id="EN-US_TOPIC_0000001188323704__p19846742258">Value range: an integer ranging from 1 to 1024</p>
<div class="note" id="EN-US_TOPIC_0000001188323704__note32011266415"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="EN-US_TOPIC_0000001188323704__p02019268413">The <strong id="EN-US_TOPIC_0000001188323704__b21279473574519">filesize</strong> parameter is valid only for the write-only HDFS foreign table in ORC format.</p>
</div></div>
</li><li id="EN-US_TOPIC_0000001188323704__li10634202212434"><strong id="EN-US_TOPIC_0000001188323704__b13761647182511">compression</strong><p id="EN-US_TOPIC_0000001188323704__p157219443509">(Optional) Specifies the compression mode of ORC files. This syntax is available only for the write-only foreign table.</p>
<p id="EN-US_TOPIC_0000001188323704__p1728715317504">Value range: <strong id="EN-US_TOPIC_0000001188323704__b5108962204519">zlib</strong>, <strong id="EN-US_TOPIC_0000001188323704__b21460021554519">snappy</strong>, and <strong id="EN-US_TOPIC_0000001188323704__b6836815764519">lz4</strong>. The default value is <strong id="EN-US_TOPIC_0000001188323704__b6309813484519">snappy</strong>.</p>
</li><li id="EN-US_TOPIC_0000001188323704__li16723183074313"><strong id="EN-US_TOPIC_0000001188323704__b5759137441226">version</strong><p id="EN-US_TOPIC_0000001188323704__p63121479433">(Optional) Specifies the ORC version number. This syntax is available only for the write-only foreign table.</p>
<p id="EN-US_TOPIC_0000001188323704__p11527451104713">Value range: Only <strong id="EN-US_TOPIC_0000001188323704__b10062233184519">0.12</strong> is supported. The default value is <strong id="EN-US_TOPIC_0000001188323704__b13225014484519">0.12</strong>.</p>
</li><li id="EN-US_TOPIC_0000001188323704__li1831945420718"><strong id="EN-US_TOPIC_0000001188323704__b18133749122511">dataencoding</strong><p id="EN-US_TOPIC_0000001188323704__p11198153517818">(Optional) Specifies the data encoding of the data table to be exported when the database encoding is different from the data encoding of the data table. For example, the database encoding is Latin-1, but the data encoding of the exported data table is in UTF-8 format. If this parameter is not specified, the database encoding is used by default. This syntax is valid only for the write-only HDFS foreign table.</p>
<p id="EN-US_TOPIC_0000001188323704__p16272113181718">Value range: data encoding types supported by the database encoding</p>
<div class="note" id="EN-US_TOPIC_0000001188323704__note1867518620215"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="EN-US_TOPIC_0000001188323704__p66751367215">The <strong id="EN-US_TOPIC_0000001188323704__b16203632374519">dataencoding</strong> parameter is valid only for the write-only HDFS foreign table in ORC format.</p>
</div></div>
</li></ul>
</li><li id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_li18119205351513"><strong id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_b101191753151519">Other parameters in the syntax</strong><p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p2011985361516">Other parameters are optional and can be configured as required. In this example, they do not need to be configured. For details, see <strong id="EN-US_TOPIC_0000001188323704__b17778611724519">CREATE FOREIGN TABLE (SQL on Hadoop or OBS)</strong>.</p>
<p id="EN-US_TOPIC_0000001188323704__en-us_topic_0102810709_p5898193171031">Based on the preceding settings, the command for creating the foreign table is as follows:</p>
<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001188323704__screen0768104386"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
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<span class="c1">-- Create an OBS foreign table that does not contain partition columns. The foreign server associated with the table is hdfs_server, the format of the file on HDFS corresponding to the table is ORC, and the data storage path on OBS is /user/hive/warehouse/product_info_orc/.</span>
<span class="k">CREATE</span><span class="w"> </span><span class="k">FOREIGN</span><span class="w"> </span><span class="k">TABLE</span><span class="w"> </span><span class="n">product_info_ext_obs</span>
<span class="p">(</span>
<span class="w"> </span><span class="n">product_price</span><span class="w"> </span><span class="nb">integer</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_id</span><span class="w"> </span><span class="nb">char</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_time</span><span class="w"> </span><span class="nb">date</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_level</span><span class="w"> </span><span class="nb">char</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_name</span><span class="w"> </span><span class="nb">varchar</span><span class="p">(</span><span class="mi">200</span><span class="p">)</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_type1</span><span class="w"> </span><span class="nb">varchar</span><span class="p">(</span><span class="mi">20</span><span class="p">)</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_type2</span><span class="w"> </span><span class="nb">char</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_monthly_sales_cnt</span><span class="w"> </span><span class="nb">integer</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_comment_time</span><span class="w"> </span><span class="nb">date</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_comment_num</span><span class="w"> </span><span class="nb">integer</span><span class="w"> </span><span class="p">,</span>
<span class="w"> </span><span class="n">product_comment_content</span><span class="w"> </span><span class="nb">varchar</span><span class="p">(</span><span class="mi">200</span><span class="p">)</span><span class="w"> </span>
<span class="p">)</span><span class="w"> </span><span class="n">SERVER</span><span class="w"> </span><span class="n">obs_server</span><span class="w"> </span>
<span class="k">OPTIONS</span><span class="w"> </span><span class="p">(</span>
<span class="n">format</span><span class="w"> </span><span class="s1">'orc'</span><span class="p">,</span><span class="w"> </span>
<span class="n">foldername</span><span class="w"> </span><span class="s1">'/user/hive/warehouse/product_info_orc/'</span><span class="p">,</span>
<span class="w"> </span><span class="n">compression</span><span class="w"> </span><span class="s1">'snappy'</span><span class="p">,</span>
<span class="w"> </span><span class="k">version</span><span class="w"> </span><span class="s1">'0.12'</span>
<span class="p">)</span><span class="w"> </span><span class="k">Write</span><span class="w"> </span><span class="k">Only</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
</li></ul>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dws_04_0159.html">Exporting ORC Data to MRS</a></div>
</div>
</div>
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