Re: Zeroing single tuple in data file.

And the \d+ on users?  Also would you mind pasting the output exactly as it is appears on screen, assuming it is in English?

I'm trying to build up a picture here of how many problems you have and whether they can be fixed conventionally with the database interface or whether you need to start messing around with the data files.  If the case is the latter, you won't be able to just remove the single rows that are causing the problem - you will loose full pages.  It could be that it is actually a corrupt index rather than the table - in this case the \d+ will help me assist you in disabling that index.

You can take a look at this guide if you suspect block corruption -

http://blog.endpoint.com/2010/06/tracking-down-database-corruption-with.html

BUT I would try everything else first.  You will loose data if you follow that approach (but it may not be avoidable).  If your database is a production one (I don't know the environment you are using this database in) then this would need to be a last ditch attempt to salvage otherwise lost data.  In any case it sounds like you have some kind of hardware issue and should consider a full backup restore if possible and then afterwards running frequent ANALYZE; commands to quickly detect further corruptions.