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<h2 class="title" style="clear: both"><a id="miscdiff"></a>Miscellaneous Differences</h2>
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<p>
The following miscellaneous differences also exist between
the BDB SQL interface and SQLite:
</p>
<div class="itemizedlist">
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<li>
<p>
The BDB SQL interface does not support the <code class="literal">IMMEDIATE</code> keyword
(<code class="literal">BEGIN IMMEDIATE</code> behaves just like <code class="literal">BEGIN</code>).
</p>
</li>
<li>
<p>
When an exclusive transaction is active, it will
block any new transactions from beginning (they
will be blocked during their first operation until
the exclusive transactions commits or aborts).
Non-exclusive transactions that are active when the
exclusive transaction begins will not be able to
execute any more operations without being blocked
until the exclusive transactions finishes.
</p>
</li>
<li>
<p>
Enabling MVCC mostly disables exclusive
transactions. Exclusive transactions can still be
used, but they will run concurrently with regular
transactions, even ones that write to the database.
The only advantage of exclusive transactions in
this case is that two exclusive transactions will
be forced to run in serial, and that if an
exclusive transaction and non-exclusive transaction
experience deadlock, then the non-exclusive
transaction will always be the transaction forced
to release its locks.
</p>
<p>
For more information on MVCC and snapshot
isolation, see
<a class="xref" href="mvcc.html" title="Using Multiversion Concurrency Control">Using Multiversion Concurrency Control</a>
</p>
</li>
<li>
<p>
There are differences in how the two products work
in a concurrent application that will cause the
BDB SQL interface to deadlock where SQLite would result in a
different error. This is because the products use
different locking paradigms. See
<a class="xref" href="lockingnotes.html" title="Chapter 2. Locking Notes">Locking Notes</a>
for more information.
</p>
</li>
<li>
<p>
The BDB SQL does not call the busy callback when a
session attempts to operate the same database page
that another session has locked. It blocks
instead. This means that the functions
<code class="literal">sqlite3_busy_handler</code> and
<code class="literal">sqlite3_busy_timeout</code> are not
effective in BDB SQL.
</p>
</li>
<li>
<p>
The BDB SQL does not support two phase commit across databases.
Attaching to multiple databases can lead to inconsistency
after recovery and undetected deadlocks when
accessing multiple databases from concurrent transactions
in different order. Hence, applications must ensure that
they access databases in the same order in any transaction
that spans multiple databases. Else, a deadlock can
occur that causes threads to block, and the deadlock will
not be detected by Berkeley DB.
</p>
</li>
<li>
<p>
In BDB SQL, when two sessions accessing the same database
perform conflicting operations on the same page, one session
will be blocked until the conflicting operations are resolved.
For example,
</p>
<p><span class="bold"><strong>Session 1:</strong></span>
</p>
<pre class="programlisting">dbsql> insert into a values (4);
dbsql> begin;
dbsql> insert into a values (5); </pre>
<p>
<span class="bold"><strong>Session 2:</strong></span>
</p>
<pre class="programlisting">dbsql> select * from a;
</pre>
<p>
What happens here is that Session 2 is blocked until Session 1 commits the transaction.
</p>
<p><span class="bold"><strong>Session 1:</strong></span>
</p>
<pre class="programlisting">dbsql> commit;
</pre>
<p>
<span class="bold"><strong>Session 2:</strong></span>
</p>
<pre class="programlisting">dbsql> select * from a;
4
5
</pre>
<p>
Under such situations in SQLite, operations poll
instead of blocking, and a callback is used to
determine whether to continue polling.
</p>
</li>
<li>
<p>
By default, you always only have a single database
file when you use BDB SQL interface SQL, just as you do
when you use SQLite. However, you can configure
BDB SQL interface at compile time to create one BDB SQL interface
database file for each SQL table that you
create. How to perform this configuration is
described in the <em class="citetitle">Berkeley DB Installation and Build Guide</em>.
</p>
</li>
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