It's clone of ZBXNEXT-1894 (I cannot reopen and attach patches there).
I propose two patches for zabbix 2.1.4.
First one changes logic of processing trigger expressions: "true or unknown" is now "true"; "false and unknown" is "false", not "unknown". It's not affect unsupported items, only undefined expressions such as "min(300)" when no data exists during last 300 seconds.
Second patch is more complex, it changes structures and algorithm of processing expressions. In addition to logic from first patch it does not evaluate trigger subexpressions (does not preform extra requests to database) if result of the whole expression is already known. For example, it's not needed to evaluate expressions {exp2}, {exp3} and {exp4} in trigger "{exp1}>0 | {exp2}>0 | {exp3}>0 | {exp4}>0" if value of {exp1} is positive. This should increase effectivity of the server.

The patched version works good on my tests but I do not think the patches are ready for applying to trunk, they were not tested hard enough. Then, calculated items are not affected.
You can consider it as explanaition what I meant in ZBXNEXT-1894.