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| <h1 class="title topictitle1" id="ariaid-title1">NUM_NODES Query Option</h1> |
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| <div class="body conbody"> |
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| <p class="p"> |
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| Limit the number of nodes that process a query, typically during debugging. |
| </p> |
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| <p class="p"> |
| <strong class="ph b">Type:</strong> numeric |
| </p> |
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| <p class="p"> |
| <strong class="ph b">Allowed values:</strong> Only accepts the values 0 |
| (meaning all nodes) or 1 (meaning all work is done on the coordinator node). |
| </p> |
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| <p class="p"> |
| <strong class="ph b">Default:</strong> 0 |
| </p> |
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| <p class="p"> |
| <strong class="ph b">Usage notes:</strong> |
| </p> |
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| <p class="p"> |
| Setting <code class="ph codeph">NUM_NODES</code> to 1 disables multithreading, i.e. if |
| <code class="ph codeph">MT_DOP</code> is greater than 1, it is effectively reduced to 1. |
| </p> |
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| <p class="p"> |
| If you are diagnosing a problem that you suspect is due to a timing issue due to |
| distributed query processing, you can set <code class="ph codeph">NUM_NODES=1</code> to verify |
| if the problem still occurs when all the work is done on a single node. |
| </p> |
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| <p class="p"> |
| You might set the <code class="ph codeph">NUM_NODES</code> option to 1 briefly, during |
| <code class="ph codeph">INSERT</code> or <code class="ph codeph">CREATE TABLE AS SELECT</code> statements. Normally, |
| those statements produce one or more data files per data node. If the write operation |
| involves small amounts of data, a Parquet table, and/or a partitioned table, the default |
| behavior could produce many small files when intuitively you might expect only a single |
| output file. <code class="ph codeph">SET NUM_NODES=1</code> turns off the <span class="q">"distributed"</span> aspect of |
| the write operation, making it more likely to produce only one or a few data files. |
| </p> |
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| <div class="note warning"><span class="warningtitle">Warning:</span> |
| <p class="p"> |
| Because this option results in increased resource utilization on a single host, |
| it could cause problems due to contention with other Impala statements or |
| high resource usage. Symptoms could include queries running slowly, exceeding the memory limit, |
| or appearing to hang. Use it only in a single-user development/test environment; |
| <strong class="ph b">do not</strong> use it in a production environment or in a cluster with a high-concurrency |
| or high-volume or performance-critical workload. |
| </p> |
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| </div> |
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| </div> |
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| <div class="related-links"> |
| <div class="familylinks"> |
| <div class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_set.html">SET Statement</a></div> |
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