Indexing + performance

Dear Friends,
     I have a table with 50 lakhs records, the table has more then 10 fields, i have primary key, i have select query with count(*) without any condition, it takes 17 seconds.

  I have another one query which will do joins with other small tables, it takes 47 seconds to give output, the result has 2 lakhs records. the indexing is not used.  I have created one index with one field ( which i used in this query, the field value has duplicates also ).

 Now the cpu usage is 4.4%, and waiting for io process .

 i have checked the above in the top command.

 If i delete that index, then it works fine, now the cpu usage takes more then 70%, and waiting shows less then 30%.

Can Anyone explain, Why postgres behaves like this.


Thanks....
     


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With Best Regards,
Petchimuthulingam S