Here is a bash program that loads your sample data into a table 'data_locks_keys' and then from that table extracts the 'single' values of column 'key' (with the first associated value of 'source') into a derived table 'data_locks_keys_distinct'.

Takes 1m 30s on 9.5dev postgres (low-end desktop). 

#!/bin/sh

# wget http://white-barn.com/tmp/data_locks_keys.zip

time unzip -p data_locks_keys.zip \
  | psql -c "
    drop table if exists data_locks_keys;
    create table data_locks_keys(key text, source text);
    copy data_locks_keys from stdin with (
      format csv,
      delimiter E'|',
      header FALSE
    );
"
#  main table data_locks_keys now has 9,197,129 rows

echo "
create table data_locks_keys_distinct
as
    select distinct on (key) key, source
    from data_locks_keys
;
" | psql

# derived table data_locks_keys_distinct has 3,692,089 rows (with colu
+mn 'key' now unique)

[download]

I'm really getting curious whether this DISTINCT ON statement isn't what you are looking for...

In reply to Re: Contemplating some set comparison tasks by erix
in thread Contemplating some set comparison tasks by dwhite20899

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