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    <p>I would create a History table for your tasks table. Same structure as tasks + a new field named previousId. This would hold the previous change id, so you can go back an forth through different changes (undo/redo).</p> <p>Why a new History table? For a simple reason: do not overload tasks table with things that it was not designed for.</p> <p>As for space, in the History, instead of a Memo, use a binary format and zip the content of the text you want to store. Don't try to detect changes. You will run into a buggy code which will result in frustration and wasted time...</p> <p>Optimization: Even better, you may keep only three columns in History table: 1. taskId (foreign key to tasks) 2. data - a binary field. Before saving in the History table, create an XML string holding only the fields that have changed. 3. previousId (will help maintain a queue of changes and allow navigation back and forth)</p> <p>As for data field, create an XML string like this:</p> <pre><code>&lt;task&gt; &lt;title&gt;Title was changed&lt;/title&gt; &lt;date_added&gt;2011-03-26 01:29:22&lt;date_added&gt; &lt;/task&gt; </code></pre> <p>This will basically tell you that this time you changed only the title and the date_added fields.</p> <p>After the XML string is built, just zip it if you want and store it into History table's data field.</p> <p>XML will also allow for flexibility. If you add / remove a field in tasks table, you don't need to update the History table, too. So this way the structure of the tasks table and History table are decoupled so you don't need to update two tables each time.</p> <p>PS: don't forget to add some indexes to quickly navigate through the history table. Fields to be indexed: taskId and previousId as you will need fast queries against this table.</p> <p>Hope this helps.</p>
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    1. COBy the way, compression will reduce your text size up to 5% of the original text. A common value is around 10%, but if you have common, repeating words you get even better compression.
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