.tip

Mar 8, 2026 · one of the small things

awk '!seen[$0]++' dedupes without sorting

sort -u changes the order of the input. This keeps the first copy of each line, in the order the lines arrived.

cat urls.txt | awk '!seen[$0]++'

The first time a line shows up, seen[$0]++ is zero, so ! makes it true and awk prints the line. Every time after that it's at least one, and nothing gets printed.

shell

~/tips/today — 2026-10-19

~cat tips/today

Never build an index on a live table without CONCURRENTLY

postgresqlmigrations

A plain CREATE INDEX takes a lock that blocks every write to the table for the whole build. On a big table that means minutes of outage that look like a hung app.

CREATE INDEX CONCURRENTLY orders_customer_idx ON orders (customer_id);

It's slower and can't run inside a transaction, so you'll have to tell your migration tool. If it fails halfway, it leaves an invalid index behind, so check pg_index.indisvalid and drop it before you retry.

↵ open y yesterday l ls /tips

esc close