We have:
zabbix=# select count(*)from items;
652688
zabbix=# select count(*)from hosts;
10249
zabbix=# select count(*)from triggers;
40436
zabbix=# select count(*)from functions;
60380

In history sync executing function DCmass_update_triggers(history, history_num) where for every function executing query:
select distinct i.itemid,i.key_,h.host,i.type,i.history,i.lastvalue,i.prevvalue,i.hostid,i.value_type,i.delta,i.prevorgvalue,i.lastclock,i.units,i.multiplier,i.formula,i.status,i.valuemapid,i.trends,i.data_type,f.function,f.parameter,h.status from hosts h,items i,functions f where i.hostid=h.hostid and i.itemid=f.itemid and f.functionid=<functionid>

zabbix=# explain select distinct i.itemid,i.key_,h.host,i.type,i.history,i.lastvalue,i.prevvalue,i.hostid,i.value_type,i.delta,i.prevorgvalue,i.lastclock,i.units,i.multiplier,i.formula,i.status,i.valuemapid,i.trends,i.data_type,f.function,f.parameter,h.status from hosts h,items i,functions f where i.hostid=h.hostid and i.itemid=f.itemid and f.functionid=36251;
QUERY PLAN
-----------------------------------------------------------------------------------------------------
HashAggregate (cost=9.13..9.13 rows=1 width=122)
-> Nested Loop (cost=0.00..9.08 rows=1 width=122)
-> Nested Loop (cost=0.00..8.82 rows=1 width=103)
-> Index Scan using functions_pkey on functions f (cost=0.00..4.26 rows=1 width=14)
Index Cond: (functionid = 36251)
-> Index Scan using items_pkey on items i (cost=0.00..4.55 rows=1 width=97)
Index Cond: (i.itemid = f.itemid)
-> Index Scan using hosts_pkey on hosts h (cost=0.00..0.26 rows=1 width=27)
Index Cond: (h.hostid = i.hostid)

Cost for thise query very small, but query executing for every! functionid and blocking rows from items,triggers,hosts tables!

bigest triggers use 'last' function, why need executing thise query? item last value exists in zabbix cache (also trigger expression, items properties).

i think evaluate 'last' function can doing without any queries and maybe some other trigger functions