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| |
| <h1 class="title topictitle1" id="ariaid-title1">SCAN_BYTES_LIMIT Query Option (<span class="keyword">Impala 3.1</span> or higher only)</h1> |
| |
| |
| |
| |
| |
| <div class="body conbody"> |
| |
| <p class="p"> |
| The <code class="ph codeph">SCAN_BYTES_LIMIT</code> query option sets a limit on the bytes scanned by |
| HDFS and HBase SCAN operations. If a query is still executing when the query’s |
| coordinator detects that it has exceeded the limit, the query is terminated with an error. |
| The option is intended to prevent runaway queries that scan more data than is intended. |
| </p> |
| |
| <p class="p"> |
| For example, an Impala administrator could set a default value of |
| <code class="ph codeph">SCAN_BYTES_LIMIT=100GB</code> for a resource pool to automatically kill queries |
| that scan more than 100 GB of data (see |
| <a class="xref" href="https://impala.apache.org/docs/build/html/topics/impala_admission.html" target="_blank">Impala |
| Admission Control and Query Queuing</a> for information about default query options). |
| If a user accidentally omits a partition filter in a <code class="ph codeph">WHERE</code> clause and |
| runs a large query that scans a lot of data, the query will be automatically terminated |
| after it scans more data than the <code class="ph codeph">SCAN_BYTES_LIMIT</code>. |
| </p> |
| |
| <div class="p"> |
| You can override the default value per-query or per-session, in the same way as other |
| query options, if you do not want the default <code class="ph codeph">SCAN_BYTES_LIMIT</code> value to |
| apply to a specific query or session. |
| <div class="note note note_note"><span class="note__title notetitle">Note:</span> |
| <ul class="ul"> |
| <li dir="ltr" class="li"> |
| <p dir="ltr" class="p"> |
| Only data actually read from the underlying storage layer is counted towards the |
| limit. E.g. Impala’s Parquet scanner employs several techniques to skip over |
| data in a file that is not relevant to a specific query, so often only a fraction |
| of the file size is counted towards <code class="ph codeph">SCAN_BYTES_LIMIT</code>. |
| </p> |
| </li> |
| |
| <li dir="ltr" class="li"> |
| <p dir="ltr" class="p"> |
| As of Impala 3.1, bytes scanned by Kudu tablet servers are not counted towards the |
| limit. |
| </p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| |
| <p class="p"> |
| Because the checks are done periodically, the query may scan over the limit at times. |
| </p> |
| |
| <p class="p"> |
| <strong class="ph b">Syntax:</strong> <code class="ph codeph">SET SCAN_BYTES_LIMIT=bytes;</code> |
| </p> |
| |
| <p class="p"> |
| <strong class="ph b">Type:</strong> numeric |
| </p> |
| |
| <div class="p"> |
| <strong class="ph b">Units:</strong> |
| <ul class="ul"> |
| <li class="li"> |
| A numeric argument represents memory size in bytes. |
| </li> |
| |
| <li class="li"> |
| Specify a suffix of <code class="ph codeph">m</code> or <code class="ph codeph">mb</code> for megabytes. |
| </li> |
| |
| <li class="li"> |
| Specify a suffix of <code class="ph codeph">g</code> or <code class="ph codeph">gb</code> for gigabytes. |
| </li> |
| |
| <li class="li"> |
| If you specify a suffix with unrecognized formats, subsequent queries fail with an |
| error. |
| </li> |
| </ul> |
| </div> |
| |
| <p class="p"> |
| <strong class="ph b">Default:</strong> <code class="ph codeph">0</code> (no limit) |
| </p> |
| |
| <p class="p"> |
| <strong class="ph b">Added in:</strong> <span class="keyword">Impala 3.1</span> |
| </p> |
| |
| </div> |
| |
| <nav role="navigation" 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></div></nav></article></main></body></html> |
