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new c49d819 Added MySQL-specific instructions for iterated queries.
c49d819 is described below
commit c49d8193a66af6c46cea674223afe6d2f37a12d6
Author: Michael Gentry <blackn...@gmail.com>
AuthorDate: Thu Apr 16 21:05:34 2020 -0400
Added MySQL-specific instructions for iterated queries.
---
src/main/site/content/docs/4.1/cayenne-guide.html | 40 +++++++++++++++++++++--
1 file changed, 38 insertions(+), 2 deletions(-)
diff --git a/src/main/site/content/docs/4.1/cayenne-guide.html
b/src/main/site/content/docs/4.1/cayenne-guide.html
index d2d3f1c..528b856 100644
--- a/src/main/site/content/docs/4.1/cayenne-guide.html
+++ b/src/main/site/content/docs/4.1/cayenne-guide.html
@@ -2444,10 +2444,46 @@ for(DataRow row : rows) {
<h4 id="iterated-queries"><a class="anchor"
href="#iterated-queries"></a>2.8.4. Iterated Queries</h4>
<div class="paragraph">
<p>While contemporary hardware may easily allow applications to fetch
hundreds of thousands or even millions of objects into memory, it doesn’t mean
this is always a good idea to do so. You can optimize processing of very large
result sets with two techniques discussed in this and the following chapter -
iterated and paginated queries.</p>
- </div>
+ </div>
<div class="paragraph">
<p>Iterated query is not actually a special query. Any selecting query
can be executed in iterated mode by an ObjectContext. ObjectContext creates an
object called <code>ResultIterator</code> that is backed by an open ResultSet.
Iterator provides constant memory performance for arbitrarily large ResultSets.
This is true at least on the Cayenne end, as JDBC driver may still decide to
bring the entire ResultSet into the JVM memory.</p>
- </div>
+ </div>
+ <div class="admonitionblock note">
+ <table>
+ <tbody>
+ <tr>
+ <td class="icon"> <i class="fa fa-info-circle fa-2x"
title="Note"></i> </td>
+ <td class="content">
+ <div>
+ The MySQL JDBC driver is known to pull the entire
ResultSet into memory.
+ In order to get the driver to stream the results,
append the JDBC parameter
+ <code class="language-java java"
data-lang="java">useCursorFetch=true</code>
+ to the JDBC URL.
+ </div>
+ <br/>
+ <div>
+ A fetch size must also be set when doing the query
using
+ <code class="language-java java"
data-lang="java">ObjectSelect.statementFetchSize(..)</code>,
+ for example:
+ </div>
+ <div class="listingblock">
+ <div class="content">
+ <pre class="highlight" style="margin-bottom:
0px">
+ <code class="language-Java Java"
data-lang="Java" style="font-size: 120%">ObjectSelect.query(queryClass)
+ .statementFetchSize(50000)
+ .iterate(objectContext, cayenneObject -> ...</code>
+ </pre>
+ </div>
+ </div>
+ <div>
+ Adjust the value of the fetch size to fit
requirements. A larger value is more
+ performant, but consumes more memory as well.
+ </div>
+ </td>
+ </tr>
+ </tbody>
+ </table>
+ </div>
<div class="paragraph">
<p>Data is read from ResultIterator one row/object at a time until it is
exhausted. There are two styles of accessing ResultIterator - direct access
which requires explicit closing to avoid JDBC resources leak, or a callback
that lets Cayenne handle resource management. In both cases iteration can be
performed using "for" loop, as ResultIterator is "Iterable".</p>
</div>
