Planner ignoring to use INDEX SCAN

Hello All,

I have two table:

master_table:
pk_id bigint primary key,
name text

child_table :
chk_id bigint,
pk_id bigint
mob varchar(15)

foreign key pk_id to master_table
index on pk_id of child

query which was taking seconds on the join of these two table suddenly started taking 20/25 min

child table has @ 100M rows

The only deifference was I dropped the foreign key pointing to parent pk_id after this the planner is ignoring to use index scan on child table and doing seq scan.

Is it because I dropped foreign key?

Thanks in advance

With regards
Ashish...

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