[skyde]

facebook data model is a Graph where each row store one object “comment” or one association “comment is with post id” between objects .

They made an query and indexing system on top of it to make it fast called TAO.

Without it you need to send a distinct SQL query pet parent object to get list of associated child object which would be awfuly slow.

[radicality]

Non-tao use cases of mysql at FB also cannot use FK constraints (or ‘triggers’).

[skyde]

by "no FK contraints" do you mean "no join using index" or you simply mean foreign key violation is not checked.