For those who have a similar problem that I have;

I recently added a new column (Col_A) to a table and then a new index using the new column and a previously existing column ( indx=Col_A,Col_B).

I started to get "No more data to read from socket error" when I try to select a third column directly or via group function while using values corresponding with the new index:

select Col_C from [table] where Col_A= [value1] and Col_B=[value2]

select distinct Col_C from [table] where Col_A= [value1] and Col_B=[value2]

select count(*) from [table] where Col_A= [value1] and Col_B=[value2] and Col_C =[value3]

All these variations caused the aforementioned error and forces me to reconnect my editor to the oracle db.

it was fixed when I added Col_C into the index or created a new index which includes Col_A,Col_B and Col_C.

So this is not a definitive solution but an example that indicates a problem with index creation/update process of the oracle db and the effects it might have on constructing select sentences. This might provide your DB Admin a more precise starting point in solving the issue.

I hope this might be of help to someone who is having a similar problem as I am. Cheers.