Although kind of a special case (1):

>       1) MySQL table metadata operations get sluggish when the server hits
>       a couple of thousands tables.  All table metadata is stored in a
>       non-transactional engine (MyISAM) so full-table locks cannot be
>       avoided;

I'd throw in database schema upgrades, which are even worse. The last
update script took "forever" for 18k users (we cancelled the upgrade
after some hours), even after putting in some optimizations we had a
downtime of multiple hours.

I'd guess running the schema change would have been finished within
seconds or minutes with only few tables.

RDBMS are made for large amounts of data in few tables, not a few rows
in thousands of them.

-- 
Jens Erat
Universität Konstanz
Kommunikations-, Infomations-, Medienzentrum (KIM)
Abteilung Basisdienste
D-78457 Konstanz
Mail: [email protected]

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