This script works in 2 parts.

1. First it will generate from the osm.pbf file the list with all the admin_level=4 (al_4)

Then it will remove the ones that are not closed. Then it will make a polygon from each al_4, using the ST_BuildArea

After it will have a inner join that will search for all the nodes and ways that are a restaurant.

2. Then, using st_contains we are searching to see in what polygon each restaurant fits.

Took 4 seconds to do it for Romania

You can find the query here https://github.com/baditaflorin/osm-postgis-scripts/blob/master/statistics/count_number_of_restaurants_inside_each_al_4.sql

Admin_level_4	Count
Municipiul București	505
Cluj	249
Constanța	217
Iași	155
Prahova	128
Brașov	124
Timiș	121
Mureș	109
Hunedoara	88
Sibiu	78
Arad	74
Galați	68
Buzău	56
Neamț	53
Harghita	53
Dolj	52
Argeș	47
Bihor	41
Covasna	38
Ilfov	36
Brăila	35
Vâlcea	33
Alba	32
Maramureș	29
Suceava	28
Bacău	28
Mehedinți	28
Caraș Severin	26
Călărași	22
Tulcea	22
Olt	20
Satu Mare	19
Gorj	18
Dâmbovița	16
Giurgiu	13
Ialomița	13
Botoșani	13
Bistrița-Năsăud	12
Vaslui	11
Vrancea	11
Teleorman	7
Sălaj	5