Today Wikidot was off-line for about 5 minutes due to database operations blocking access to page revision table. We added a column to mark tag changes in page revisions. This took very long time unexpectedly (on the test server it was a quick operation and we do have a full copy of Wikidot database there). Also it caused SELECT operations on that table to block until the ALTER operation finished.
Wikidot service is up again and we don't expect any major problems.
BTW, what we did was a preparation to implementing a wish to store tag changes in revision data rather than relying on revision comments only.
The photo comes from Flickr