1. A computer-implemented method of placing materialized query tables (MQTs) in a distributed database system for improving load distribution and reducing network latency, said method comprising: inputting, by a user, to a materialized query table advisor (MQTA) of said distributed databases system, data placement destinations for MQTs in a frontend database and backend databases, a workload comprising a database update log and a read workload, and a simulated catalog of backend databases; deriving, by said MQTA, candidate MQTs based on common parts of query statements; calculating, by said MQTA, a total benefit for each instance of said candidate MQTs of all read queries of said workload in terms of resource time by comparing an estimated query processing time with and without said candidate MQTs, and a total overhead for refreshing said each candidate MQT for all write queries in said database update log in terms of said resource time; deriving, by said MQTA, dependencies among said candidate MQTs, wherein a dependency indicates how multiple candidate MQTs are co-used in a single query statement; deriving, by said MQTA, a total benefit for each dependency in terms of said resource time; outputting, by said MQTA, to a data placement advisor (DPA), a query statement, an MQT identification, and a total benefit, corresponding to said workload; inputting, by said user, to said DPA, database sizes allocated for said candidate MQTs in said frontend database and said backend databases, wherein a user specifies a space limit of said database sizes for any of said distributed database system, said frontend database, and said backend databases; measuring, by said DPA, a synchronization cost for each of said candidate MQTs at said frontend database and said backend database, in terms of said resource time at said frontend to determine a total synchronization cost; performing, by said DPA, a what-if data placement analysis comprising: creating a ranked list of said candidate MQTs, based on return on investment (ROI), wherein said ROI is determined by dividing a net benefit, equal to said total benefit minus said total overhead minus said total synchronization cost, of each of said candidate MQTs by each of said candidate MOT's size; for each said dependency, creating a virtual caching unit (VCU), determining said ROI for each said VCU, and inserting said VCU in said ranked list; and selecting from said ranked list said candidate MQT or VCU having a highest ROI and fitting said space limit, removing said selected MQT or VCU from said ranked list, and inserting said selected MQT or VCU into a recommended MQT list; and re-calculating ROIs of said ranked list, based on subsumption of a candidate MQT by a VCU, and selecting from said ranked list said candidate MQT or VCU having a highest ROI and fitting said space limit, and inserting said selected MQT or VCU into said recommended MQT list, until said space limit is exceeded; and using, by a data placement manager, said recommended MQT list, to reduce network latency in said distributed database system, by creating MQTs at said backend databases from said recommended MQT list; creating frontend MQTs, based on created backend MQTs; and synchronizing said created frontend MQTs with said created backend MQTs.