Why is the query not using the index for sorting?

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I have a table with a number of columns.
 
I perform
 
Select *
from table
order by a,b
 
There is an index on a,b which is clustered (as well as indexes on a and b alone).
I have issued the cluster and anyalze commands.
 
Nevertheless, PostgreSQL performs a Sequential Scan on the table and then performs a sort.
Am I missing something?
 
Jonathan Blitz

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