[GENERAL] Slow queries on very big (and partitioned) table

Hu guys,

we have a test machine with Postgresql 9.6.1 an about 800.000.000 record in a table.
Table is partitioned by day, with indexes on partitioned table.

Also a simple query (for example i want to search log occurred in a specific day), is immediate in tha partitioned
table(table_2017_02_15) but *extremely* slow in global table. 

Where am i wrong?
Shall i create global index?

Thank you!
/F