My point was specifically about each RDBMS' peculiarity when it comes to writing SQL for performance rather than functionality. I would expect a good programmer to have a broad knowledge of what functionality is available for each RDBMS they use on a regular basis, but I certainly wouldn't expect such a programmer to go to the lengths of understanding how each RDBMS optimises their SQL. For instance, I don't know DB2 at all, but I have it on very good authority that its optimizer changes with every release - keeping track of such things is surely not a job for a programmer. For another instance, a few years ago I was was called in to a large company to fix the problems that they were having with their Telephone Banking system - their daily batch update was taking 27 hours! (do the maths :) A couple of small tweaks later (changed an 'or' to a 'union all' and modified the column ordering in a couple of indexes) and the batch load time was reduced to 42 minutes. The point of that little story was that the original SQL was written by Oracle Corp consultants charging GBP150p.h. - programmers who should know how the Oracle RDBMS works :)
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