Tags field is a text field that stores a delimited list of tags for a question. I believe this was done to improve query performance when searching for questions with certain tags -- normalized schema vs. full-text index? I dunno which would win, though I suppose the latter since that appears to be what's in use.
PostTags table exists only in Data Explorer (you'll notice it isn't in the data dump directly) as a normalized version of a question's tags, so you can do
JOINs in your queries, instead of doing text searches on the
When running queries in Data Explorer, use the
PostTags table exclusively.
Data Explorer runs on SQL Azure so there's no full-text index on the Data Explorer runs on SQL 2008, but there isn't a full-text index on the