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richvdh
2021-06-11 16:14:17 +00:00
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@@ -267,6 +267,42 @@ has had all background updates run.</p>
</code></pre>
<p>NB at the time of writing, this script predates the split into separate <code>state</code>/<code>main</code>
databases so will require updates to handle that correctly.</p>
<h2 id="boolean-columns"><a class="header" href="#boolean-columns">Boolean columns</a></h2>
<p>Boolean columns require special treatment, since SQLite treats booleans the
same as integers.</p>
<p>There are three separate aspects to this:</p>
<ul>
<li>
<p>Any new boolean column must be added to the <code>BOOLEAN_COLUMNS</code> list in
<code>scripts/synapse_port_db</code>. This tells the port script to cast the integer
value from SQLite to a boolean before writing the value to the postgres
database.</p>
</li>
<li>
<p>Before SQLite 3.23, <code>TRUE</code> and <code>FALSE</code> were not recognised as constants by
SQLite, and the <code>IS [NOT] TRUE</code>/<code>IS [NOT] FALSE</code> operators were not
supported. This makes it necessary to avoid using <code>TRUE</code> and <code>FALSE</code>
constants in SQL commands.</p>
<p>For example, to insert a <code>TRUE</code> value into the database, write:</p>
<pre><code class="language-python">txn.execute(&quot;INSERT INTO tbl(col) VALUES (?)&quot;, (True, ))
</code></pre>
</li>
<li>
<p>Default values for new boolean columns present a particular
difficulty. Generally it is best to create separate schema files for
Postgres and SQLite. For example:</p>
<pre><code class="language-sql"># in 00delta.sql.postgres:
ALTER TABLE tbl ADD COLUMN col BOOLEAN DEFAULT FALSE;
</code></pre>
<pre><code class="language-sql"># in 00delta.sql.sqlite:
ALTER TABLE tbl ADD COLUMN col BOOLEAN DEFAULT 0;
</code></pre>
<p>Note that there is a particularly insidious failure mode here: the Postgres
flavour will be accepted by SQLite 3.22, but will give a column whose
default value is the <strong>string</strong> <code>&quot;FALSE&quot;</code> - which, when cast back to a boolean
in Python, evaluates to <code>True</code>.</p>
</li>
</ul>
</main>