Hive Partitioning Ineffective while using Hive Functions in Queries - hive

Question

I've been working on Hive partitioning from the past few days. Here is an example I created :-
Table - Transactions (Non - Partitioned Managed Table):
CREATE TABLE TRANSACTIONS (
txnno INT,
txndate STRING,
custid INT,
amount DOUBLE,
category STRING,
product STRING,
city STRING,
state STRING,
spendby STRING)
row format delimited fields terminated by ',';
Loaded the data inside this table using the load command.
Created another table as follows :-
Table - Txnrecordsbycat (Partitioned Managed Table):
CREATE TABLE TXNRECORDSBYCAT(txnno INT, txndate STRING, custno INT, amount DOUBLE, product STRING, city STRING, state STRING, spendby STRING)
partitioned by (category STRING)
clustered by (state) INTO 10 buckets
row format delimited fields terminated by ',';
Used the following query to load the data from Transactions table to Txnrecordsbycat table.
FROM TRANSACTIONS txn INSERT OVERWRITE TABLE TXNRECORDSBYCAT PARTITION(category) select txn.txnno,txn.txndate,txn.custid, txn.amount, txn.product,txn.city, txn.state, txn.spendby,txn.category DISTRIBUTE BY CATEGORY;
Now as long as I'm firing simple queires like select * from Transactions and select * from trxrecordsbycat, I can see my queries being efficient (i.e take less execution time) on the partitioned table as compared to non-partitioned table.
However, as soon as my queries become a little complex, something like select count(*) from table, the query becomes less efficient (i.e takes more time) on partitioned table.
Any idea whey this might be happening ?
Many Thanks for help.