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<title>SQL Hints</title>
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</div><hr /><div id="mysqlnd-ms.quickstart.sqlhints" class="section">
<h2 class="title">SQL Hints</h2>
<p class="para">
SQL hints can force a query to choose a specific server from the connection pool.
It gives the plugin a hint to use a designated server, which can solve
issues caused by connection switches and connection state.
</p>
<p class="para">
SQL hints are standard compliant SQL comments. Because
SQL comments are supposed to be ignored by SQL processing systems, they
do not interfere with other programs such as the MySQL Server, the MySQL Proxy,
or a firewall.
</p>
<p class="para">
Three SQL hints are supported by the plugin: The
<strong><code>MYSQLND_MS_MASTER_SWITCH</code></strong> hint makes the plugin run a
statement on the master, <strong><code>MYSQLND_MS_SLAVE_SWITCH</code></strong>
enforces the use of the slave, and
<strong><code>MYSQLND_MS_LAST_USED_SWITCH</code></strong> will run a statement on
the same server that was used for the previous statement.
</p>
<p class="para">
The plugin scans the beginning of a statement for the existence of an SQL
hint. SQL hints are only recognized if they appear at the beginning of
the statement.
</p>
<p class="para">
<div class="example" id="example-1762">
<p><strong>Example #1 Plugin config with one slave and one master</strong></p>
<div class="example-contents">
<div class="inicode"><pre class="inicode">{
"myapp": {
"master": {
"master_0": {
"host": "localhost",
"socket": "\/tmp\/mysql.sock"
}
},
"slave": {
"slave_0": {
"host": "192.168.2.27",
"port": "3306"
}
}
}
}</pre>
</div>
</div>
</div>
</p>
<p class="para">
<div class="example" id="example-1763">
<p><strong>Example #2 SQL hints to prevent connection switches</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$mysqli </span><span style="color: #007700">= new </span><span style="color: #0000BB">mysqli</span><span style="color: #007700">(</span><span style="color: #DD0000">"myapp"</span><span style="color: #007700">, </span><span style="color: #DD0000">"username"</span><span style="color: #007700">, </span><span style="color: #DD0000">"password"</span><span style="color: #007700">, </span><span style="color: #DD0000">"database"</span><span style="color: #007700">);<br />if (</span><span style="color: #0000BB">mysqli_connect_errno</span><span style="color: #007700">())<br /> </span><span style="color: #FF8000">/* Of course, your error handling is nicer... */<br /> </span><span style="color: #007700">die(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">mysqli_connect_errno</span><span style="color: #007700">(), </span><span style="color: #0000BB">mysqli_connect_error</span><span style="color: #007700">()));<br /><br /></span><span style="color: #FF8000">/* Connection 1, connection bound SQL user variable, no SELECT thus run on master */<br /></span><span style="color: #007700">if (!</span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">query</span><span style="color: #007700">(</span><span style="color: #DD0000">"SET @myrole='master'"</span><span style="color: #007700">)) {<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">errno</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">error</span><span style="color: #007700">);<br />}<br /><br /></span><span style="color: #FF8000">/* Connection 1, run on master because of SQL hint */<br /></span><span style="color: #007700">if (!(</span><span style="color: #0000BB">$res </span><span style="color: #007700">= </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">query</span><span style="color: #007700">(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"/*%s*/SELECT @myrole AS _role"</span><span style="color: #007700">, </span><span style="color: #0000BB">MYSQLND_MS_LAST_USED_SWITCH</span><span style="color: #007700">)))) {<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">errno</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">error</span><span style="color: #007700">);<br />} else {<br /> </span><span style="color: #0000BB">$row </span><span style="color: #007700">= </span><span style="color: #0000BB">$res</span><span style="color: #007700">-></span><span style="color: #0000BB">fetch_assoc</span><span style="color: #007700">();<br /> </span><span style="color: #0000BB">$res</span><span style="color: #007700">-></span><span style="color: #0000BB">close</span><span style="color: #007700">();<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"@myrole = '%s'\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$row</span><span style="color: #007700">[</span><span style="color: #DD0000">'_role'</span><span style="color: #007700">]);<br />}<br /></span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">close</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
@myrole = 'master'
</pre></div>
</div>
</div>
</p>
<p class="para">
In the above example, using <strong><code>MYSQLND_MS_LAST_USED_SWITCH</code></strong> prevents
session switching from the master to a slave when running the <em>SELECT</em>
statement.
</p>
<p class="para">
SQL hints can also be used to run <em>SELECT</em> statements
on the MySQL master server. This may be desired if the MySQL slave servers
are typically behind the master, but you need current data from the cluster.
</p>
<p class="para">
In version 1.2.0 the concept of a service level has been introduced to address
cases when current data is required. Using a service level requires less attention
and removes the need of using SQL hints for this use case. Please, find more
information below in the service level and consistency section.
</p>
<p class="para">
<div class="example" id="example-1764">
<p><strong>Example #3 Fighting replication lag</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$mysqli </span><span style="color: #007700">= new </span><span style="color: #0000BB">mysqli</span><span style="color: #007700">(</span><span style="color: #DD0000">"myapp"</span><span style="color: #007700">, </span><span style="color: #DD0000">"username"</span><span style="color: #007700">, </span><span style="color: #DD0000">"password"</span><span style="color: #007700">, </span><span style="color: #DD0000">"database"</span><span style="color: #007700">);<br />if (!</span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">)<br /> </span><span style="color: #FF8000">/* Of course, your error handling is nicer... */<br /> </span><span style="color: #007700">die(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">mysqli_connect_errno</span><span style="color: #007700">(), </span><span style="color: #0000BB">mysqli_connect_error</span><span style="color: #007700">()));<br /><br /></span><span style="color: #FF8000">/* Force use of master, master has always fresh and current data */<br /></span><span style="color: #007700">if (!</span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">query</span><span style="color: #007700">(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"/*%s*/SELECT critical_data FROM important_table"</span><span style="color: #007700">, </span><span style="color: #0000BB">MYSQLND_MS_MASTER_SWITCH</span><span style="color: #007700">))) {<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">errno</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">error</span><span style="color: #007700">);<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="para">
A use case may include the creation of tables on a slave.
If an SQL hint is not given, then the plugin will send <em>CREATE</em>
and <em>INSERT</em> statements to the master. Use the
SQL hint <strong><code>MYSQLND_MS_SLAVE_SWITCH</code></strong> if you want to
run any such statement on a slave, for example, to build temporary
reporting tables.
</p>
<p class="para">
<div class="example" id="example-1765">
<p><strong>Example #4 Table creation on a slave</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$mysqli </span><span style="color: #007700">= new </span><span style="color: #0000BB">mysqli</span><span style="color: #007700">(</span><span style="color: #DD0000">"myapp"</span><span style="color: #007700">, </span><span style="color: #DD0000">"username"</span><span style="color: #007700">, </span><span style="color: #DD0000">"password"</span><span style="color: #007700">, </span><span style="color: #DD0000">"database"</span><span style="color: #007700">);<br />if (!</span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">)<br /> </span><span style="color: #FF8000">/* Of course, your error handling is nicer... */<br /> </span><span style="color: #007700">die(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">mysqli_connect_errno</span><span style="color: #007700">(), </span><span style="color: #0000BB">mysqli_connect_error</span><span style="color: #007700">()));<br /><br /></span><span style="color: #FF8000">/* Force use of slave */<br /></span><span style="color: #007700">if (!</span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">query</span><span style="color: #007700">(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"/*%s*/CREATE TABLE slave_reporting(id INT)"</span><span style="color: #007700">, </span><span style="color: #0000BB">MYSQLND_MS_SLAVE_SWITCH</span><span style="color: #007700">))) {<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">errno</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">error</span><span style="color: #007700">);<br />}<br /></span><span style="color: #FF8000">/* Continue using this particular slave connection */<br /></span><span style="color: #007700">if (!</span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">query</span><span style="color: #007700">(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"/*%s*/INSERT INTO slave_reporting(id) VALUES (1), (2), (3)"</span><span style="color: #007700">, </span><span style="color: #0000BB">MYSQLND_MS_LAST_USED_SWITCH</span><span style="color: #007700">))) {<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">errno</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">error</span><span style="color: #007700">);<br />}<br /></span><span style="color: #FF8000">/* Don't use MYSQLND_MS_SLAVE_SWITCH which would allow switching to another slave! */<br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">$res </span><span style="color: #007700">= </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">query</span><span style="color: #007700">(</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"/*%s*/SELECT COUNT(*) AS _num FROM slave_reporting"</span><span style="color: #007700">, </span><span style="color: #0000BB">MYSQLND_MS_LAST_USED_SWITCH</span><span style="color: #007700">))) {<br /> </span><span style="color: #0000BB">$row </span><span style="color: #007700">= </span><span style="color: #0000BB">$res</span><span style="color: #007700">-></span><span style="color: #0000BB">fetch_assoc</span><span style="color: #007700">();<br /> </span><span style="color: #0000BB">$res</span><span style="color: #007700">-></span><span style="color: #0000BB">close</span><span style="color: #007700">();<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"There are %d rows in the table 'slave_reporting'"</span><span style="color: #007700">, </span><span style="color: #0000BB">$row</span><span style="color: #007700">[</span><span style="color: #DD0000">'_num'</span><span style="color: #007700">]);<br />} else {<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"[%d] %s\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">errno</span><span style="color: #007700">, </span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">error</span><span style="color: #007700">);<br />}<br /></span><span style="color: #0000BB">$mysqli</span><span style="color: #007700">-></span><span style="color: #0000BB">close</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="para">
The SQL hint <strong><code>MYSQLND_MS_LAST_USED</code></strong> forbids switching a
connection, and forces use of the previously used connection.
</p>
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