<p>Simple one really.  In SQL, if I want to search a text field for a couple of characters, I can do:</p>

<pre><code>SELECT blah FROM blah WHERE blah LIKE '%text%'
</code></pre>

<p>The documentation for App Engine makes no mention of how to achieve this, but surely it's a common enough problem?</p>

BigTable, which is the database back end for App Engine, will scale to millions of records. Due to this, App Engine will not allow you to do any query that will result in a table scan, as performance would be dreadful for a well populated table.
In other words, every query must use an index. This is why you can only do
```
=
```
,
```
>
```
and
```
<
```
queries. (In fact you can also do
```
!=
```
but the API does this using a a combination of
```
>
```
and
```
<
```
queries.)  This is also why the development environment monitors all the queries you do and automatically adds any missing indexes to your
```
index.yaml
```
file.
There is no way to index for a
```
LIKE
```
query so it's simply not available.
Have a watch of
this Google IO session
for a much better and more detailed explanation of this.