SQL engines are often slower than what?
Than engines designed for massive parallelism in dealing with workloads which can be effectually processed in parallel.
Operating on what hypothetical database schema with how many records spread across how many tables?
Generally NoSQL engines use schema on read techniques not schema on write. The table structure comes during the read. To get some sort of fair comparison something like a typical star schema with a much too large fact table (think billions or trillions of rows) and a half dozen dimension tables.
Or if you really want to make it worse. The same query where the table is getting 1m writes / second and you want an accurate stream.
SQL engines have problems with massive parallelism? Why? Which ones?
Because SQL by its nature operates on the table not the individual rows. Older database technologies that were row oriented like what you see on a mainframe on in SaS work better when the ratio of table size to computation speed is low. Today because disk storage size per dollar has gone up so fast, we disk we face many of the same problems systems in the 1980s faced with tape.
And the next question is pretty much all of them. The big data SQL engines have the least problems though and via. their execution plans turning into map-reduces might present a viable long term solution.
How well do you *really* know SQL in general and the capabilities of different database engines in particular?
Assume I don't know anything. Oracle, which has the best engine and SQL people on the planet has a guide for hybridization to handle things their engine can't handle well. IBM which probably comes in second and invented the relational database produces their own Hadoop / R to handle queries that DB2 (which is BTW far better than Oracle at stream) can't handle. Teradata's engine which was originally written specifically for larger amounts of data for a decade has had specific features of another subsystem to do enhanced big data, they also have guides for hybridization for things even their enhanced engine can't handle And Microsoft which writes the 3rd most popular engine has spent many millions on hybridization strategies. Enterprise DB (postgres) fully supports the IBM strategy.
I don't know anyone in the space who does agree with the /. "SQL can do everything" attitude.
but that portion off the article was ridiculous, and thus far all of the comments in support of it have demonstrated a similar lack of familiarity with actual databases, their operation, or performance tuning.
The article was ridiculous I said as much in another response. However the comment I was responding to went much too far in the other direction. As for performance tuning -- performance tuning is designed to avoid full table scans and expensive joins. To goal of many hybridization strategies is to take a raw data flow and convert it into a relational ETL using a big data engine which can take advantage of indexing and a better execution plan. It doesn't do much good when the initial goal is to do a full table scan.