The OP in that thread was able to index a 100MB file with ~6 million lines in seconds and pick out random lines in milliseconds by pre-indexing every 1000 lines.

If your 1TB file had avg 80 characters per line, you'd have 13 billion lines. If you indexed every 1000 lines, you could index 13 million lines. At 8 bytes per index point, that would be 100MB for the index.

You pre-index by reading the entire file, and every 1000 lines, write the file position to your index file.

Then, for example, to find line 12345, you divide that by 1000, and read the 12'th index position in the index (12*8). Then seek to that position in the big file and start reading. After 345 lines you're at your position.

You can tune the 1000 to trade off speed and space.