Like everywhere in Italy, there are a bunch of big, ornate and mostly rococo Catholic churches (https://en.wikipedia.org/wiki/Rococo), in Bologna. The most grand is Basilca of San Petronio, (whoever he was), the patron saint of the city, https://en.wikipedia.org/wiki/San_Petronio,_Bologna . It is far bigger than the official metropolitan cathedral. In terms of interior volume, and it is the sixth largest church in Europe.
We ended up in Bologna by chance because air fares from London were cheaper than flying into Florence. I'm glad we came. Definitely worth a visit and OMG you eat well here.