We use postgresql for 2 internal tracs on 0.12, one is pretty large (6,000 tickets; at least 50 ticket updates per workday and hundreds of wiki pages). The only issues we've had have been occasional plugins that have like one or two lines of SQL that's invalid for Postgresql. We also had one semi-frustrating error message that's since been fixed, http://trac.edgewall.org/ticket/9400. Otherwise it's been a very satisfying experience.
I have a bunch of trac instances running with pgsql, one with 4300 tickets, 100 users and 32K svn changesets. It has basically worked fine, with two kinds of minor hiccups in plugins: lack of begin/end (for transactions) interaction with autocommit sloppy use of types in sql both were easy to resolve and most plugins seem ok with pgsql now.
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