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HHH-7746 - docs

This commit is contained in:
Strong Liu 2013-03-14 15:43:20 +08:00
parent 3c6f27a5dd
commit 06e3313f23
1 changed files with 52 additions and 5 deletions
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documentation/src/main/docbook/manual/en-US/content/performance.xml
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@ -456,8 +456,55 @@ Cat fritz = (Cat) iter.next();</programlisting>
<programlisting role="XML">&lt;class name="Person" batch-size="10"&gt;...&lt;/class&gt;</programlisting>
<para>Hibernate will now execute only three queries: the pattern is 10,
10, 5.</para>
<para>With this <literal>batch-size</literal> specified, Hibernate will now execute queries on demand when need to access the
uninitialized proxy, as above, but the difference is that instead of querying the exactly proxy entity that
being accessed, it will query more Person's owner at once, so, when accessing other person's owner, it may
already been initialized by this batch fetch with only a few ( much less than 25) queries will be executed.
</para>
<para>This behavior is controlled by the <literal>batch-size</literal> and batch fetch style configuration.
The batch fetch style configuration ( <literal>hibernate.batch_fetch_style</literal> ) is a new performance
improvement since 4.2.0, there are 3 different strategies provided, which is <literal>legacy</literal>,
<literal>padded</literal> and <literal>dynamic</literal>.
</para>
<itemizedlist>
<listitem>
<para><literal>LEGACY</literal></para>
<para>The legacy algorithm where we keep a set of pre-built batch sizes based on
<classname>org.hibernate.internal.util.collections.ArrayHelper#getBatchSizes</classname>.
Batches are performed using the next-smaller pre-built batch size from the number of existing batchable identifiers.</para>
<para>In the above example, with a batch-size setting of 25 the pre-built batch sizes would be [25, 12, 10, 9, 8, 7, .., 1].</para>
<para>And since there are 25 persons' owner to be initialized, then only one query will be executed using these 25 owners' identifier.</para>
<para>But in another case, suppose there are only 24 persons, there will be 3 queries (12, 10, 2) will
be executed to go through all person's owner, and the query will looks like :
</para>
<programlisting role="SQL">select * from owner where id in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
select * from owner where id in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
select * from owner where id in (?, ?)
</programlisting>
</listitem>
<listitem>
<para><literal>PADDED</literal></para>
<para> This is kind of similar with the legacy algorithm, it uses the pre-build batch sizes based on same
<classname>org.hibernate.internal.util.collections.ArrayHelper#getBatchSizes</classname>. The difference
is that here hibernate will use the next-bigger batch size and pads the extra identifier placeholders.</para>
<para>So, using the same example above, initializing 25 persons the query would be same as above,
only 1 query will be executed to batch query all the owners.
</para>
<para>However, the attempt to batch load 24 owners would result just a single batch of size 25, the
identifiers to load would be "padded" (aka, repeated) to make up the difference.
</para>
</listitem>
<listitem>
<para><literal>DYNAMIC</literal></para>
<para>Dynamically builds its SQL based on the actual number of available ids. Does still limit to the batch-size defined on the entity.</para>
</listitem>
</itemizedlist>
<para>You can also enable batch fetching of collections. For example, if
each <literal>Person</literal> has a lazy collection of
@ -474,8 +521,8 @@ Cat fritz = (Cat) iter.next();</programlisting>
&lt;/set&gt;
&lt;/class&gt;</programlisting>
<para>With a <literal>batch-size</literal> of 3, Hibernate will load 3,
3, 3, 1 collections in four <literal>SELECT</literal>s. Again, the value
<para>For example, with a <literal>batch-size</literal> of 3 and using <literal>legacy</literal> batch style,
Hibernate will load 3, 3, 3, 1 collections in four <literal>SELECT</literal>s. Again, the value
of the attribute depends on the expected number of uninitialized
collections in a particular <literal>Session</literal>.</para>