Incidentally (and fyi only), as part of an initiative to move our developer-related documentation to its own dedicated site (i.e. https://developer.atlassian.com), the latest (and currently maintained) version of this 'Database Schema' document is now located here:
Having said that, I don't believe there have been many updates wrt the schema itself (and hence this page) since the release of JIRA 4.1.
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