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Lu, Huayi ef0ada5a59 DWS DEV 20240716 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>
2024-11-02 09:07:47 +00:00

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<a name="EN-US_TOPIC_0000001510522565"></a><a name="EN-US_TOPIC_0000001510522565"></a>
<h1 class="topictitle1">Skew Hints</h1>
<div id="body1543482317963"><div class="section" id="EN-US_TOPIC_0000001510522565__section290819468377"><h4 class="sectiontitle">Function</h4><p id="EN-US_TOPIC_0000001510522565__p164213954412">Theses hints specify redistribution keys containing skew data and skew values, and are used to optimize redistribution involving Join or HashAgg.</p>
</div>
<div class="section" id="EN-US_TOPIC_0000001510522565__section7104551113714"><h4 class="sectiontitle">Precautions</h4><ul id="EN-US_TOPIC_0000001510522565__ul2019913451381"><li id="EN-US_TOPIC_0000001510522565__li1919994533819">Skew hints are used only if redistribution is required and the specified skew information matches the redistribution information.</li><li id="EN-US_TOPIC_0000001510522565__li81992458381">Skew hints are controlled by the GUC parameter <a href="dws_04_0909.html#EN-US_TOPIC_0000001510522673__section1211182712176">skew_option</a>. If the parameter is disabled, skew hints cannot be used for solving skew.</li><li id="EN-US_TOPIC_0000001510522565__li8199184515385">Currently, skew hints support only the table relationships of the ordinary table and subquery types. Hints can be specified for base tables, subqueries, and <strong id="EN-US_TOPIC_0000001510522565__b09644618206">WITH ... AS</strong> clauses. Unlike other hints, a subquery can be used in skew hints regardless of whether it is pulled up.</li><li id="EN-US_TOPIC_0000001510522565__li13199114533814">Use an alias (if any) to specify a table where data skew occurs.</li><li id="EN-US_TOPIC_0000001510522565__li16878659143810">You can use a name or an alias to specify a skew column as long as it is not ambiguous. The columns in skew hints cannot be expressions. If data skew occurs in the redistribution that uses an expression as a redistribution key, set the redistribution key as a new column and specify the column in skew hints.</li><li id="EN-US_TOPIC_0000001510522565__li18781459183813">The number of skew values must be an integer multiple of the number of columns. Skew values must be grouped based on the column sequence, with each group containing a maximum of 10 values. You can specify duplicate values to group skew columns having different number of skew values. For example, the <strong id="EN-US_TOPIC_0000001510522565__b2260164012201">c1</strong> and <strong id="EN-US_TOPIC_0000001510522565__b1489164219206">c2</strong> columns of the <strong id="EN-US_TOPIC_0000001510522565__b1793024912200">t1</strong> table contains skew data. The skew value of the <strong id="EN-US_TOPIC_0000001510522565__b24781915182113">c1</strong> column is <strong id="EN-US_TOPIC_0000001510522565__b889271762113">a1</strong>, and the skew values of the <strong id="EN-US_TOPIC_0000001510522565__b231982720216">c2</strong> column are <strong id="EN-US_TOPIC_0000001510522565__b53261030172112">b1</strong> and <strong id="EN-US_TOPIC_0000001510522565__b116761332192110">b2</strong>. In this case, the skew hint is <strong id="EN-US_TOPIC_0000001510522565__b660935419218">skew(t1 (c1 c2) ((a1 b1)(a1 b2)))</strong>. <strong id="EN-US_TOPIC_0000001510522565__b7218191551717">(a1 b1)</strong> is a value group, where <strong id="EN-US_TOPIC_0000001510522565__b1821951511719">NULL</strong> is allowed as a skew value. Each hint can contain a maximum of 10 groups and the number of groups should be an integer multiple of the number of columns.</li><li id="EN-US_TOPIC_0000001510522565__li20878155993810">In the redistribution optimization of Join, a skew value must be specified for skew hints. The skew value can be left empty for HashAgg.</li><li id="EN-US_TOPIC_0000001510522565__li1087825923816">If multiple tables, columns, or values are specified, separate items of the same type with spaces.</li><li id="EN-US_TOPIC_0000001510522565__li68780597389">The type of skew values cannot be forcibly converted in hints. To specify a string, enclose it with single quotation marks (' ').</li></ul>
</div>
<div class="section" id="EN-US_TOPIC_0000001510522565__section530131664410"><h4 class="sectiontitle">Syntax</h4><ul id="EN-US_TOPIC_0000001510522565__ul4920124610417"><li id="EN-US_TOPIC_0000001510522565__li1192094616410">Specify single-table skew.<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen67171647938"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span></pre></div></td><td class="code"><div><pre><span></span><span class="n">skew</span><span class="p">([</span><span class="o">@</span><span class="n">block_name</span><span class="p">]</span><span class="w"> </span><span class="k">table</span><span class="w"> </span><span class="p">(</span><span class="k">column</span><span class="p">)</span><span class="w"> </span><span class="p">[(</span><span class="n">value</span><span class="p">)])</span>
</pre></div></td></tr></table></div>
</div>
</li><li id="EN-US_TOPIC_0000001510522565__li209211046745">Specify intermediate result skew.<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen14923573417"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span></pre></div></td><td class="code"><div><pre><span></span><span class="n">skew</span><span class="p">([</span><span class="o">@</span><span class="n">block_name</span><span class="p">]</span><span class="w"> </span><span class="p">(</span><span class="n">join_rel</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="k">column</span><span class="p">)</span><span class="w"> </span><span class="p">[(</span><span class="n">value</span><span class="p">)])</span>
</pre></div></td></tr></table></div>
</div>
</li></ul>
</div>
<div class="section" id="EN-US_TOPIC_0000001510522565__section41303128143838"><h4 class="sectiontitle">Parameter Description</h4><ul id="EN-US_TOPIC_0000001510522565__ul7813152810126"><li id="EN-US_TOPIC_0000001510522565__li184869398337"><em id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001510402197_i172885331829">block_name</em> indicates the block name of the statement block. For details, see <a href="dws_04_0456.html#EN-US_TOPIC_0000001460722632__li99021444551">block_name</a>.</li><li id="EN-US_TOPIC_0000001510522565__li98131128181220"><strong id="EN-US_TOPIC_0000001510522565__b179821130201818">table</strong> specifies the table where skew occurs.<div class="note" id="EN-US_TOPIC_0000001510522565__note1743015338294"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><ul id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_ul114831571316"><li id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_li134831971819">The syntax format of the table is as follows:<p id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_p6483117519"><a name="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_li134831971819"></a><a name="en-us_topic_0000001460722632_li134831971819"></a>[schema.]table[@block_name]</p>
<p id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_p204832718119">The table name can contain the schema name or block name before the subquery statement block is promoted. If the subquery statement block is optimized and rewritten by the optimizer, the value of <strong id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_b1442317392810">block_name</strong> is different from that of <strong id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_b152968102815">block_name</strong> in leading.</p>
</li><li id="EN-US_TOPIC_0000001510522565__en-us_topic_0000001460722632_li3483157118">If a table has an alias, the alias is preferentially used to represent the table.</li></ul>
</div></div>
</li><li id="EN-US_TOPIC_0000001510522565__li948751319356"><strong id="EN-US_TOPIC_0000001510522565__b18488191393511">join_rel</strong> specifies two or more joined tables. For example, <strong id="EN-US_TOPIC_0000001510522565__b1851613545912">(t1 t2)</strong> indicates that the result of joining <strong id="EN-US_TOPIC_0000001510522565__b13734195115915">t1</strong> and <strong id="EN-US_TOPIC_0000001510522565__b1041517499592">t2</strong> tables contains skew data.</li><li id="EN-US_TOPIC_0000001510522565__li451542111339"><strong id="EN-US_TOPIC_0000001510522565__b1851519213337">column</strong> specifies one or more columns where skew occurs.</li><li id="EN-US_TOPIC_0000001510522565__li51430225337"><strong id="EN-US_TOPIC_0000001510522565__b514392283314">value</strong> specifies one or more skew values.</li></ul>
</div>
<p id="EN-US_TOPIC_0000001510522565__p11571335134810">Example:</p>
<ul id="EN-US_TOPIC_0000001510522565__ul14835192215556"><li id="EN-US_TOPIC_0000001510522565__li108352227558">Specify single-table skew.<p id="EN-US_TOPIC_0000001510522565__p183121031115512"><a name="EN-US_TOPIC_0000001510522565__li108352227558"></a><a name="li108352227558"></a>Each skew hint describes the skew information of one table relationship. To describe the skews of multiple table relationships in a query, specify multiple skew hints.</p>
<p id="EN-US_TOPIC_0000001510522565__p17972182411553">Skew hints have the following formats:</p>
<ul id="EN-US_TOPIC_0000001510522565__ul13579114317550"><li id="EN-US_TOPIC_0000001510522565__li10579104320556">One skew value in one column: <strong id="EN-US_TOPIC_0000001510522565__b11856203073917">skew(t (c1) (v1))</strong><p id="EN-US_TOPIC_0000001510522565__p93051478551">Description: The <strong id="EN-US_TOPIC_0000001510522565__b1871133464017">v1</strong> value in the <strong id="EN-US_TOPIC_0000001510522565__b95301144124019">c1</strong> column of the <strong id="EN-US_TOPIC_0000001510522565__b736852114017">t</strong> table relationship causes skew in query execution.</p>
</li><li id="EN-US_TOPIC_0000001510522565__li357994313556">Multiple skew values in one column: <strong id="EN-US_TOPIC_0000001510522565__b1177912342423">skew(t (c1) (v1 v2 v3 ...))</strong><p id="EN-US_TOPIC_0000001510522565__p1871394845515">Description: Values including <strong id="EN-US_TOPIC_0000001510522565__b12349134115426">v1,</strong> <strong id="EN-US_TOPIC_0000001510522565__b691914434425">v2</strong>, and <strong id="EN-US_TOPIC_0000001510522565__b13759494429">v3</strong> in the <strong id="EN-US_TOPIC_0000001510522565__b874692544318">c1</strong> column of the <strong id="EN-US_TOPIC_0000001510522565__b13001131184317">t</strong> table relationship cause skew in query execution.</p>
</li><li id="EN-US_TOPIC_0000001510522565__li20579104314552">Multiple columns, each having one skew value: <strong id="EN-US_TOPIC_0000001510522565__b893116312444">skew(t (c1 c2) (v1 v2))</strong><p id="EN-US_TOPIC_0000001510522565__p3727114905518">Description: The <strong id="EN-US_TOPIC_0000001510522565__b621812515453">v1</strong> value in the <strong id="EN-US_TOPIC_0000001510522565__b18218112518451">c1</strong> column and the <strong id="EN-US_TOPIC_0000001510522565__b2087418438451">v2</strong> value in the <strong id="EN-US_TOPIC_0000001510522565__b6874104374517">c2</strong> column of the <strong id="EN-US_TOPIC_0000001510522565__b1621952513452">t</strong> table relationship cause skew in query execution.</p>
</li><li id="EN-US_TOPIC_0000001510522565__li1257913434557">Multiple columns, each having multiple skew values: <strong id="EN-US_TOPIC_0000001510522565__b247142420462">skew(t (c1 c2) ((v1 v2) (v3 v4) (v5 v6) ...))</strong><p id="EN-US_TOPIC_0000001510522565__p10891125045516">Description: Values including <strong id="EN-US_TOPIC_0000001510522565__b7846153611460">v1,</strong> <strong id="EN-US_TOPIC_0000001510522565__b11846436124617">v3</strong>, and <strong id="EN-US_TOPIC_0000001510522565__b15847113614468">v5</strong> in the <strong id="EN-US_TOPIC_0000001510522565__b5847236174614">c1</strong> column and values including <strong id="EN-US_TOPIC_0000001510522565__b1145145234618">v2,</strong> <strong id="EN-US_TOPIC_0000001510522565__b1645115219468">v4</strong>, and <strong id="EN-US_TOPIC_0000001510522565__b1245195215468">v6</strong> in the <strong id="EN-US_TOPIC_0000001510522565__b1746195218463">c2</strong> column of the <strong id="EN-US_TOPIC_0000001510522565__b6847103694614">t</strong> table relationship cause skew in query execution.</p>
<div class="notice" id="EN-US_TOPIC_0000001510522565__note19227161962210"><span class="noticetitle"><img src="public_sys-resources/notice_3.0-en-us.png"> </span><div class="noticebody"><p id="EN-US_TOPIC_0000001510522565__p208541936144516">In the last format, parentheses for skew value groups can be omitted, for example, <strong id="EN-US_TOPIC_0000001510522565__b34119561542">skew(t (c1 c2) (v1 v2 v3 v4 v5 v6 ...))</strong>. In a skew hint, either use parentheses for all skew value groups or for none of them.</p>
<p id="EN-US_TOPIC_0000001510522565__p1311818193483">Otherwise, a syntax error will be generated. For example, <strong id="EN-US_TOPIC_0000001510522565__b721034317915">skew(t (c1 c2) (v1 v2 v3 v4 (v5 v6) ...))</strong> will generate an error.</p>
</div></div>
</li></ul>
</li></ul>
<ul id="EN-US_TOPIC_0000001510522565__ul5152161214149"><li id="EN-US_TOPIC_0000001510522565__li99931711562">Specify intermediate result skew.<p id="EN-US_TOPIC_0000001510522565__p7630132275617"><a name="EN-US_TOPIC_0000001510522565__li99931711562"></a><a name="li99931711562"></a>If data skew does not occur in base tables but in an intermediate result during query execution, specify skew hints of the intermediate result to solve the skew. The format is <strong id="EN-US_TOPIC_0000001510522565__b4148653144912">skew((t1 t2) (c1) (v1))</strong>.</p>
<p id="EN-US_TOPIC_0000001510522565__p11167102515567">Description: Data skew occurs after the table relationships <strong id="EN-US_TOPIC_0000001510522565__b4759164053817">t1</strong> and <strong id="EN-US_TOPIC_0000001510522565__b8761340143816">t2</strong> are joined. The <strong id="EN-US_TOPIC_0000001510522565__b2761174043819">c1</strong> column of the <strong id="EN-US_TOPIC_0000001510522565__b2076194063819">t1</strong> table contains skew data and its skew value is <strong id="EN-US_TOPIC_0000001510522565__b9763164023811">v1</strong>.</p>
<p id="EN-US_TOPIC_0000001510522565__p59151383568"><strong id="EN-US_TOPIC_0000001510522565__b105218408521">c1</strong> can exist only in a table relationship of <strong id="EN-US_TOPIC_0000001510522565__b12398184815217">join_rel</strong>. If there is another column having the same name, use aliases to avoid ambiguity.</p>
</li></ul>
<div class="section" id="EN-US_TOPIC_0000001510522565__section99281150122819"><h4 class="sectiontitle">Suggestion</h4><ul id="EN-US_TOPIC_0000001510522565__ul19827192311618"><li id="EN-US_TOPIC_0000001510522565__li208275234618">For a multi-level query, write the hint on the layer where data skew occurs.</li><li id="EN-US_TOPIC_0000001510522565__li684316261356">For a listed subquery, you can specify the subquery name in a hint. If you know data skew occurs on which base table, directly specify the table.</li><li id="EN-US_TOPIC_0000001510522565__li19827323764">Aliases are preferred when you specify a table or column in a hint.</li></ul>
</div>
<div class="section" id="EN-US_TOPIC_0000001510522565__section780913135614"><h4 class="sectiontitle">Examples</h4><p id="EN-US_TOPIC_0000001510522565__p13554162643619">Specify single-table skew.</p>
<ul id="EN-US_TOPIC_0000001510522565__ul4727174618186"><li id="EN-US_TOPIC_0000001510522565__li185061372569">Specify hints in the original query.<p id="EN-US_TOPIC_0000001510522565__p6242133885619"><a name="EN-US_TOPIC_0000001510522565__li185061372569"></a><a name="li185061372569"></a>For example, the original query is as follows:</p>
<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen1347124316443"><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">23</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">explain</span>
<span class="k">with</span><span class="w"> </span><span class="n">customer_total_return</span><span class="w"> </span><span class="k">as</span>
<span class="p">(</span><span class="k">select</span><span class="w"> </span><span class="n">sr_customer_sk</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="n">ctr_customer_sk</span>
<span class="p">,</span><span class="n">sr_store_sk</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="n">ctr_store_sk</span>
<span class="p">,</span><span class="k">sum</span><span class="p">(</span><span class="n">SR_FEE</span><span class="p">)</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="n">ctr_total_return</span>
<span class="k">from</span><span class="w"> </span><span class="n">store_returns</span>
<span class="p">,</span><span class="n">date_dim</span>
<span class="k">where</span><span class="w"> </span><span class="n">sr_returned_date_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">d_date_sk</span>
<span class="k">and</span><span class="w"> </span><span class="n">d_year</span><span class="w"> </span><span class="o">=</span><span class="mi">2000</span>
<span class="k">group</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">sr_customer_sk</span>
<span class="p">,</span><span class="n">sr_store_sk</span><span class="p">)</span>
<span class="w"> </span><span class="k">select</span><span class="w"> </span><span class="n">c_customer_id</span>
<span class="k">from</span><span class="w"> </span><span class="n">customer_total_return</span><span class="w"> </span><span class="n">ctr1</span>
<span class="p">,</span><span class="n">store</span>
<span class="p">,</span><span class="n">customer</span>
<span class="k">where</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_total_return</span><span class="w"> </span><span class="o">&gt;</span><span class="w"> </span><span class="p">(</span><span class="k">select</span><span class="w"> </span><span class="k">avg</span><span class="p">(</span><span class="n">ctr_total_return</span><span class="p">)</span><span class="o">*</span><span class="mi">1</span><span class="p">.</span><span class="mi">2</span>
<span class="k">from</span><span class="w"> </span><span class="n">customer_total_return</span><span class="w"> </span><span class="n">ctr2</span>
<span class="k">where</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_store_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ctr2</span><span class="p">.</span><span class="n">ctr_store_sk</span><span class="p">)</span>
<span class="k">and</span><span class="w"> </span><span class="n">s_store_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_store_sk</span>
<span class="k">and</span><span class="w"> </span><span class="n">s_state</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s1">'NM'</span>
<span class="k">and</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_customer_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">c_customer_sk</span>
<span class="k">order</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">c_customer_id</span>
<span class="k">limit</span><span class="w"> </span><span class="mi">100</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000001510522565__p168241747298"><span><img id="EN-US_TOPIC_0000001510522565__image198241340293" src="figure/en-us_image_0000001460723200.png" height="223.08528900000002" width="492.1" title="Click to enlarge" class="imgResize"></span></p>
<p id="EN-US_TOPIC_0000001510522565__p1964013358219">Specify the hints of HashAgg in the inner <strong id="EN-US_TOPIC_0000001510522565__b630818333111">with</strong> clause and of the outer Hash Join. The query containing hints is as follows:</p>
<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen194951433184320"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
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<span class="normal">18</span>
<span class="normal">19</span>
<span class="normal">20</span>
<span class="normal">21</span>
<span class="normal">22</span>
<span class="normal">23</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">explain</span>
<span class="k">with</span><span class="w"> </span><span class="n">customer_total_return</span><span class="w"> </span><span class="k">as</span>
<span class="p">(</span><span class="k">select</span><span class="w"> </span><span class="cm">/*+ skew(store_returns(sr_store_sk sr_customer_sk)) */</span><span class="n">sr_customer_sk</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="n">ctr_customer_sk</span>
<span class="p">,</span><span class="n">sr_store_sk</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="n">ctr_store_sk</span>
<span class="p">,</span><span class="k">sum</span><span class="p">(</span><span class="n">SR_FEE</span><span class="p">)</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="n">ctr_total_return</span>
<span class="k">from</span><span class="w"> </span><span class="n">store_returns</span>
<span class="p">,</span><span class="n">date_dim</span>
<span class="k">where</span><span class="w"> </span><span class="n">sr_returned_date_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">d_date_sk</span>
<span class="k">and</span><span class="w"> </span><span class="n">d_year</span><span class="w"> </span><span class="o">=</span><span class="mi">2000</span>
<span class="k">group</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">sr_customer_sk</span>
<span class="p">,</span><span class="n">sr_store_sk</span><span class="p">)</span>
<span class="w"> </span><span class="k">select</span><span class="w"> </span><span class="cm">/*+ skew(ctr1(ctr_customer_sk)(11))*/</span><span class="w"> </span><span class="n">c_customer_id</span>
<span class="k">from</span><span class="w"> </span><span class="n">customer_total_return</span><span class="w"> </span><span class="n">ctr1</span>
<span class="p">,</span><span class="n">store</span>
<span class="p">,</span><span class="n">customer</span>
<span class="k">where</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_total_return</span><span class="w"> </span><span class="o">&gt;</span><span class="w"> </span><span class="p">(</span><span class="k">select</span><span class="w"> </span><span class="k">avg</span><span class="p">(</span><span class="n">ctr_total_return</span><span class="p">)</span><span class="o">*</span><span class="mi">1</span><span class="p">.</span><span class="mi">2</span>
<span class="k">from</span><span class="w"> </span><span class="n">customer_total_return</span><span class="w"> </span><span class="n">ctr2</span>
<span class="k">where</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_store_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ctr2</span><span class="p">.</span><span class="n">ctr_store_sk</span><span class="p">)</span>
<span class="k">and</span><span class="w"> </span><span class="n">s_store_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_store_sk</span>
<span class="k">and</span><span class="w"> </span><span class="n">s_state</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s1">'NM'</span>
<span class="k">and</span><span class="w"> </span><span class="n">ctr1</span><span class="p">.</span><span class="n">ctr_customer_sk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">c_customer_sk</span>
<span class="k">order</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">c_customer_id</span>
<span class="k">limit</span><span class="w"> </span><span class="mi">100</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000001510522565__p491032201810">The hints indicate that the <strong id="EN-US_TOPIC_0000001510522565__b171021514128">group by</strong> in the inner <strong id="EN-US_TOPIC_0000001510522565__b189605461215">with</strong> clause contains skew data during redistribution by HashAgg, corresponding to the original Hash Agg operators 10 and 21; and that the <strong id="EN-US_TOPIC_0000001510522565__b10833125618139">ctr_customer_sk</strong> column in the outer <strong id="EN-US_TOPIC_0000001510522565__b20386021417">ctr1</strong> table contains skew data during redistribution by Hash Join, corresponding to operator 6 in the original plan. The optimized plan is as follows:</p>
<p id="EN-US_TOPIC_0000001510522565__p15340104812278"><span><img id="EN-US_TOPIC_0000001510522565__image3340124812715" src="figure/en-us_image_0000001510284029.png" height="237.145384" width="492.1" title="Click to enlarge" class="imgResize"></span></p>
<p id="EN-US_TOPIC_0000001510522565__p35873872152236">To solve data skew in the redistribution, Hash Agg is changed to double-level Agg operators and the redistribution operators used by Hash Join are changed in the optimized plan.</p>
</li></ul>
</div>
<ul id="EN-US_TOPIC_0000001510522565__ul158125416184"><li id="EN-US_TOPIC_0000001510522565__li36611151125615">Modify the query and then specify hints.<p id="EN-US_TOPIC_0000001510522565__p145361352185616"><a name="EN-US_TOPIC_0000001510522565__li36611151125615"></a><a name="li36611151125615"></a>For example, the original query and its plan are as follows:</p>
<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen206121055105113"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">explain</span><span class="w"> </span><span class="k">select</span><span class="w"> </span><span class="k">count</span><span class="p">(</span><span class="o">*</span><span class="p">)</span><span class="w"> </span><span class="k">from</span><span class="w"> </span><span class="n">store_sales_1</span><span class="w"> </span><span class="k">group</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">round</span><span class="p">(</span><span class="n">ss_list_price</span><span class="p">);</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000001510522565__p430515361280"><span><img id="EN-US_TOPIC_0000001510522565__image8305103612280" src="figure/en-us_image_0000001510522957.png" height="55.480019000000006" width="399" title="Click to enlarge" class="imgResize"></span></p>
<p id="EN-US_TOPIC_0000001510522565__p8875256145913">Columns in hints do not support expressions. To specify hints, rewrite the query as several subqueries. The rewritten query and its plan are as follows:</p>
<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen1962513418118"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">explain</span><span class="w"> </span>
<span class="k">select</span><span class="w"> </span><span class="k">count</span><span class="p">(</span><span class="o">*</span><span class="p">)</span><span class="w"> </span>
<span class="k">from</span><span class="w"> </span><span class="p">(</span><span class="k">select</span><span class="w"> </span><span class="n">round</span><span class="p">(</span><span class="n">ss_list_price</span><span class="p">),</span><span class="n">ss_hdemo_sk</span>
<span class="k">from</span><span class="w"> </span><span class="n">store_sales_1</span><span class="p">)</span><span class="n">tmp</span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="n">ss_hdemo_sk</span><span class="p">)</span><span class="w"> </span>
<span class="k">group</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">a</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000001510522565__p1978041252917"><span><img id="EN-US_TOPIC_0000001510522565__image15779112142913" src="figure/en-us_image_0000001510163285.png" height="55.480019000000006" width="399" title="Click to enlarge" class="imgResize"></span></p>
<p id="EN-US_TOPIC_0000001510522565__p118651731845">Ensure that the service logic is not changed during the rewriting.</p>
<p id="EN-US_TOPIC_0000001510522565__p0533241750">Specify hints in the rewritten query as follows:</p>
<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen6416469111"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">explain</span><span class="w"> </span>
<span class="k">select</span><span class="w"> </span><span class="cm">/*+ skew(tmp(a)) */</span><span class="w"> </span><span class="k">count</span><span class="p">(</span><span class="o">*</span><span class="p">)</span><span class="w"> </span>
<span class="k">from</span><span class="w"> </span><span class="p">(</span><span class="k">select</span><span class="w"> </span><span class="n">round</span><span class="p">(</span><span class="n">ss_list_price</span><span class="p">),</span><span class="n">ss_hdemo_sk</span>
<span class="k">from</span><span class="w"> </span><span class="n">store_sales_1</span><span class="p">)</span><span class="n">tmp</span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="n">ss_hdemo_sk</span><span class="p">)</span><span class="w"> </span>
<span class="k">group</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">a</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000001510522565__p633451914716"><span><img id="EN-US_TOPIC_0000001510522565__image13334419573" src="figure/en-us_image_0000001460563396.png" height="81.573289" width="465.5" title="Click to enlarge" class="imgResize"></span></p>
<p id="EN-US_TOPIC_0000001510522565__p1329514175113">The plan shows that after Hash Agg is changed to double-layer Agg operators, redistributed data is greatly reduced and redistribution time shortened.</p>
<p id="EN-US_TOPIC_0000001510522565__p1452543111226">You can specify hints in columns in a subquery, for example:</p>
<div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000001510522565__screen3934194816238"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">explain</span><span class="w"> </span>
<span class="k">select</span><span class="w"> </span><span class="cm">/*+ skew(tmp(b)) */</span><span class="w"> </span><span class="k">count</span><span class="p">(</span><span class="o">*</span><span class="p">)</span><span class="w"> </span>
<span class="k">from</span><span class="w"> </span><span class="p">(</span><span class="k">select</span><span class="w"> </span><span class="n">round</span><span class="p">(</span><span class="n">ss_list_price</span><span class="p">)</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="n">ss_hdemo_sk</span>
<span class="k">from</span><span class="w"> </span><span class="n">store_sales_1</span><span class="p">)</span><span class="n">tmp</span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="n">ss_hdemo_sk</span><span class="p">)</span><span class="w"> </span>
<span class="k">group</span><span class="w"> </span><span class="k">by</span><span class="w"> </span><span class="n">a</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_0454.html">Hint-based Tuning</a></div>
</div>
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