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<h2 id="fetch">Fetch</h2>
<p>
With fetch we specify the properties that should be fetched on associated beans. Beans that
are related by <em>OneToOne, OneToMany, ManyToOne and ManyToMany</em>.
</p>
<p>
<em>Fetch</em> means we want the query to eagerly load that path and that we <em>prefer</em>
for that to happen as a <em>SQL JOIN</em>.
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="n">QCustomer</span> <span class="n">cust</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="n">QContact</span> <span class="n">cont</span> <span class="o">=</span> <span class="n">QContact</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="n">List</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">customers</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QCustomer</span><span class="o">()</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="n">cust</span><span class="o">.</span><span class="na">name</span><span class="o">,</span> <span class="n">cust</span><span class="o">.</span><span class="na">version</span><span class="o">,</span> <span class="n">cust</span><span class="o">.</span><span class="na">whenCreated</span><span class="o">)</span> <span class="c1">// root level properties</span>
<span class="o">.</span><span class="na field">contacts</span><span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="n">cont</span><span class="o">.</span><span class="na">email</span><span class="o">)</span> <span class="c1">// contacts is a OneToMany path</span>
<span class="o">.</span><span class="na field">name</span><span class="o">.</span><span class="na">istartsWith</span><span class="o">(</span><span class="s">"Rob"</span><span class="o">)</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
</div>
<p>
The resulting sql is:
</p>
<div class="syntax sql"><div class="highlight"><pre><span></span><span class="k">select</span> <span class="n">t0</span><span class="p">.</span><span class="n">id</span><span class="p">,</span> <span class="n">t0</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">t0</span><span class="p">.</span><span class="k">version</span><span class="p">,</span> <span class="n">t0</span><span class="p">.</span><span class="n">when_created</span><span class="p">,</span> <span class="c1">-- customer columns</span>
<span class="n">t1</span><span class="p">.</span><span class="n">id</span><span class="p">,</span> <span class="n">t1</span><span class="p">.</span><span class="n">email</span> <span class="c1">-- contact columns</span>
<span class="k">from</span> <span class="n">customer</span> <span class="n">t0</span>
<span class="k">left</span> <span class="k">join</span> <span class="n">contact</span> <span class="n">t1</span> <span class="k">on</span> <span class="n">t1</span><span class="p">.</span><span class="n">customer_id</span> <span class="o">=</span> <span class="n">t0</span><span class="p">.</span><span class="n">id</span>
<span class="k">where</span> <span class="k">lower</span><span class="p">(</span><span class="n">t0</span><span class="p">.</span><span class="n">name</span><span class="p">)</span> <span class="k">like</span> <span class="o">?</span> <span class="k">escape</span><span class="s1">'|'</span> <span class="k">order</span> <span class="k">by</span> <span class="n">t0</span><span class="p">.</span><span class="n">id</span>
</pre></div>
</div>
<p>
The <em>contacts</em> path is a <em>OneToMany</em> so we get a <em>left join</em> to the
contact table. We only fetch the id and email properties for the contacts.
</p>
<p>
The above query written using the query expression API is:
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="n">List</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">customers</span> <span class="o">=</span>
<span class="n">database</span><span class="o">.</span><span class="na">find</span><span class="o">(</span><span class="n">Customer</span><span class="o">.</span><span class="na">class</span><span class="o">)</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="s">"name, version, whenCreated"</span><span class="o">)</span> <span class="c1">// root level properties</span>
<span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="s">"contacts"</span><span class="o">,</span> <span class="s">"email"</span><span class="o">)</span> <span class="c1">// contacts is a OneToMany path</span>
<span class="o">.</span><span class="na">where</span><span class="o">()</span>
<span class="o">.</span><span class="na">istartsWith</span><span class="o">(</span><span class="s">"name"</span><span class="o">,</span> <span class="s">"Rob"</span><span class="o">)</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
</div>
<h2 id="fetchVariations" class="art">Fetch variations - fetch, fetchQuery, fetchCache, fetchLazy</h2>
<p>
There are 4 variations of fetch. The summary of what they are is:
</p>
<ul>
<li>fetch - fetch eagerly preferring SQL JOIN</li>
<li>fetchQuery - fetch eagerly using a separate secondary SQL query</li>
<li>fetchCache - fetch eagerly hitting the L2 cache, cache misses fetched using a separate secondary SQL query</li>
<li>fetchLazy - fetch lazy</li>
</ul>
<p>
Ebean applies <a href="#rules">rules</a> when translating ORM queries to SQL queries such that:
</p>
<ul>
<li>Ebean will not generate a SQL cartesian product</li>
<li>Ebean will honor maxRows in SQL</li>
</ul>
<p>
Ebean will automatically convert <em>fetch</em> paths into <em>fetchQuery</em> paths to ensure that it
honors these rules. In effect there is a limit on the number of OneToMany & ManyToMany paths that can
be included in a SQL query (as otherwise we get a SQL cartesian product which is not desirable).
</p>
<h2 id="fetchQuery" class="art">FetchQuery</h2>
<p>
When we specify <em>fetchQuery</em> we are telling Ebean that we want this path fetched eagerly but that
we want that to be done using a separate sql query (known as a "secondary query" or "query join").
</p>
<p>
We choose to do this on specific paths because we know the relative costs of sql joins vs query joins
based on cardinality and the relative width of the results etc.
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="n">List</span><span class="o"><</span><span class="n">Order</span><span class="o">></span> <span class="n">orders</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QOrder</span><span class="o">()</span>
<span class="o">.</span><span class="na field">customer</span><span class="o">.</span><span class="na">fetchQuery</span><span class="o">()</span> <span class="c1">// fetch customer (and children) using a "secondary query"</span>
<span class="o">.</span><span class="na field">customer</span><span class="o">.</span><span class="na field">billingAddress</span><span class="o">.</span><span class="na">fetch</span><span class="o">()</span>
<span class="o">.</span><span class="na field">customer</span><span class="o">.</span><span class="na field">shippingAddress</span><span class="o">.</span><span class="na">fetch</span><span class="o">()</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
</div>
<p>
With the above query we use <code>fetchQuery</code> to explicitly fetch the customers with their billing and
shipping addresses using a second separate sql query.
</p>
<h5>Primary query</h5>
<div class="syntax sql"><div class="highlight"><pre><span></span><span class="c1">-- "primary" query fetches the orders only</span>
<span class="k">select</span> <span class="n">t0</span><span class="p">.</span><span class="n">id</span><span class="p">,</span> <span class="p">...</span>
<span class="k">from</span> <span class="n">orders</span> <span class="n">t0</span>
</pre></div>
</div>
<h5>Secondary query (aka "query join")</h5>
<div class="syntax sql"><div class="highlight"><pre><span></span><span class="c1">-- "secondary" query fetches customers with</span>
<span class="c1">-- their billing and shipping addresses</span>
<span class="k">select</span> <span class="n">t0</span><span class="p">.</span><span class="n">id</span><span class="p">,</span> <span class="n">t0</span><span class="p">.</span><span class="n">inactive</span><span class="p">,</span> <span class="n">t0</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="p">...,</span> <span class="c1">-- customer</span>
<span class="n">t1</span><span class="p">.</span><span class="n">id</span><span class="p">,</span> <span class="n">t1</span><span class="p">.</span><span class="n">line1</span><span class="p">,</span> <span class="n">t1</span><span class="p">.</span><span class="n">line2</span><span class="p">,</span> <span class="n">t1</span><span class="p">.</span><span class="n">city</span><span class="p">,</span> <span class="p">...,</span> <span class="c1">-- customers billing address</span>
<span class="n">t2</span><span class="p">.</span><span class="n">id</span><span class="p">,</span> <span class="n">t2</span><span class="p">.</span><span class="n">line1</span><span class="p">,</span> <span class="n">t2</span><span class="p">.</span><span class="n">line2</span><span class="p">,</span> <span class="n">t2</span><span class="p">.</span><span class="n">city</span><span class="p">,</span> <span class="p">...</span> <span class="c1">-- customers shipping address</span>
<span class="k">from</span> <span class="n">customer</span> <span class="n">t0</span>
<span class="k">join</span> <span class="n">address</span> <span class="n">t1</span> <span class="k">on</span> <span class="n">t1</span><span class="p">.</span><span class="n">id</span> <span class="o">=</span> <span class="n">t0</span><span class="p">.</span><span class="n">billing_address_id</span>
<span class="k">join</span> <span class="n">address</span> <span class="n">t2</span> <span class="k">on</span> <span class="n">t2</span><span class="p">.</span><span class="n">id</span> <span class="o">=</span> <span class="n">t0</span><span class="p">.</span><span class="n">shipping_address_id</span>
<span class="k">where</span> <span class="n">t0</span><span class="p">.</span><span class="n">id</span> <span class="k">in</span> <span class="p">(</span><span class="o">?</span><span class="p">,</span> <span class="o">?</span><span class="p">,</span> <span class="p">...</span> <span class="p">)</span> <span class="c1">-- customer ids</span>
</pre></div>
</div>
<h2 id="fetchCache" class="art">FetchCache</h2>
<p>
<em>fetchCache</em> is like fetchQuery except we hit the L2 cache first.
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="n">List</span><span class="o"><</span><span class="n">Order</span><span class="o">></span> <span class="n">orders</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QOrder</span><span class="o">()</span>
<span class="o">.</span><span class="na field">customer</span><span class="o">.</span><span class="na">fetchCache</span><span class="o">()</span> <span class="c1">// Hit L2 cache for customer, for cache miss load from database</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
</div>
<p>
In the above the query fetches orders from the database. For Customer it will hit the L2 cache
for customer and for the cache misses it will load the Customers from the Database. This is performed
in batches of 100 customers at a time.
</p>
<h2 id="fetchLazy" class="art">FetchLazy</h2>
<p>
With <em>fetchLazy</em> we don't want to eagerly fetch this path BUT if it is lazy loaded we are optimising
what the lazy loading query will fetch.
</p>
<h5>e.g. lazy loading Customer only fetch the name.</h5>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="n">List</span><span class="o"><</span><span class="n">Order</span><span class="o">></span> <span class="n">orders</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QOrder</span><span class="o">()</span>
<span class="o">.</span><span class="na">fetchLazy</span><span class="o">(</span><span class="s">"customer"</span><span class="o">,</span> <span class="s">"name"</span><span class="o">)</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
<span class="o">...</span>
<span class="c1">// invoke lazy loading will only fetch customer name</span>
<span class="n">customer</span><span class="o">.</span><span class="na">getName</span><span class="o">();</span>
</pre></div>
</div>
<h5>e.g. lazy loading Customer additionally fetch the billingAddress and shippingAddress.</h5>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="n">QCustomer</span> <span class="n">cust</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="n">List</span><span class="o"><</span><span class="n">Order</span><span class="o">></span> <span class="n">orders</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QOrder</span><span class="o">()</span>
<span class="o">.</span><span class="na">fetchLazy</span><span class="o">(</span><span class="s">"customer"</span><span class="o">)</span>
<span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="s">"customer.billingAddress"</span><span class="o">)</span> <span class="c1">// fetched with lazy load of customer</span>
<span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="s">"customer.shippingAddress"</span><span class="o">)</span> <span class="c1">// fetched with lazy load of customer</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
<span class="o">...</span>
<span class="c1">// invoke lazy loading will fetch customer name + billingAddress + shippingAddress</span>
<span class="n">customer</span><span class="o">.</span><span class="na">getName</span><span class="o">();</span>
</pre></div>
</div>
<h2 id="querybean" class="art">Alias Query beans</h2>
<p>
When using query beans we use <code>alias</code> beans to provide properties for both
the <em>select</em> and <em>fetch</em>.
</p>
<div class="syntax java"><div class="highlight"><pre><span></span><span class="c1">// "alias" beans for customer and contact which</span>
<span class="c1">// we use to specify the properties to select and fetch</span>
<span class="n">QCustomer</span> <span class="n">cust</span> <span class="o">=</span> <span class="n">QCustomer</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="n">QContact</span> <span class="n">con</span> <span class="o">=</span> <span class="n">QContact</span><span class="o">.</span><span class="na">alias</span><span class="o">();</span>
<span class="c1">// using query beans</span>
<span class="n">List</span><span class="o"><</span><span class="n">Customer</span><span class="o">></span> <span class="n">customers</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">QCustomer</span><span class="o">()</span>
<span class="o">.</span><span class="na">select</span><span class="o">(</span><span class="n">cust</span><span class="o">.</span><span class="na">name</span><span class="o">,</span> <span class="n">cust</span><span class="o">.</span><span class="na">version</span><span class="o">,</span> <span class="n">cust</span><span class="o">.</span><span class="na">whenCreated</span><span class="o">)</span> <span class="c1">// customer properties only</span>
<span class="o">.</span><span class="na field">contacts</span><span class="o">.</span><span class="na">fetch</span><span class="o">(</span><span class="n">con</span><span class="o">.</span><span class="na">email</span><span class="o">)</span> <span class="c1">// contact properties only</span>
<span class="o">.</span><span class="na field">name</span><span class="o">.</span><span class="na">istartsWith</span><span class="o">(</span><span class="s">"Rob"</span><span class="o">)</span>
<span class="o">.</span><span class="na">findList</span><span class="o">();</span>
</pre></div>
</div>
<p> </p>
<h3>Kotlin alias query beans</h3>
<p>
For Kotlin the <em>alias</em> beans are a companion object of the query bean
and accessed via <code>_alias</code>. Otherwise the queries are the same in Kotlin.
</p>
<div class="syntax kotlin"><div class="highlight"><pre><span></span><span class="k">val</span> <span class="py">cust</span> <span class="p">=</span> <span class="n">QCustomer</span><span class="p">.</span><span class="n">_alias</span>
<span class="k">val</span> <span class="py">con</span> <span class="p">=</span> <span class="n">QContact</span><span class="p">.</span><span class="n">_alias</span>
</pre></div>
</div>
<h2 id="tuning" class="art">Select + Fetch = projection query tuning</h2>
<p>
Using <em>select</em> and <em>fetch</em> we are effectively tuning the query
controlling what part of the object graph is loaded.
</p>
<p>
Generally we prefer a query to only load what it needs to from the database. This gives us
the ability to reduce the cost of the query, the amount of data fetched and pulled over
the network as well as the opportunity to use covering indexes etc.
</p>
<h4>select -> "<em>graph root</em> level properties"</h4>
<p>
The <em>select</em> defines what to fetch at the <em>root level of the object graph</em>.
</p>
<h4>fetch -> "<em>graph leaf</em> properties"</h4>
<p>
The <em>fetch</em> defines what to fetch for the <em>leaves of the object graph</em>. This
translates to <em>what to fetch on the OneToOne, OneToMany, ManyToOne and ManyToMany paths</em>
of the object graph.
</p>
<p>
Note that <a href="fetchgroup">FetchGroup</a> provides an alternative to <em>select + fetch</em>.
</p>
<h2 id="rules" class="art">ORM query to SQL query rules</h2>
<p>
Ebean enforces 2 rules when it converts an ORM query into SQL queries. These rules have the effect of
limiting the number of sql joins that it will allow when translating an ORM query into SQL queries.
These 2 rules are:
</p>
<ul>
<li>Ebean will not generate a SQL cartesian product</li>
<li>Ebean will honor maxRows in SQL</li>
</ul>
<h3>Rule 1: No SQL Cartesian product</h3>
<p>
This rule has the implication that Ebean will allow at most 1 join to a <code>OneToMany</code> path
or a <code>ManyToMany</code> path. If the ORM query has more than 1 of those paths then the ORM query will
be translated and executed as multiple SQL queries.
</p>
<p>
We avoid sql cartesian product because it has a high risk of producing a very very bad sql query.
</p>
<h3>Rule 2: Always honor maxRows in SQL</h3>
<p>
This rule has the implication that Ebean can not include any joins to a <code>OneToMany</code> path
or a <code>ManyToMany</code> path if there is a <code>maxRows</code>. This is because SQL max rows
works on rows so we can't include SQL joins to paths that have a cardinality of more than 1.
</p>
<p>
Honoring maxRows is SQL is important because the database can typically give us a much better
query plan and more efficient sql queries.
</p>
<p>
Ebean detects which paths are "ToMany" paths and determines if it needs to convert any of them
from <em>fetch</em> paths to <em>fetchQuery</em> paths. Ebean firstly takes into account
<code>@FetchPreference</code> mapping and then the order in which the paths are specified in the query.
That is, generally the first ToMany path stays as <em>fetch</em> and the other ToMany paths are converted
to <em>fetchQuery</em>.
</p>
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