.tip

Mar 10, 2026 · one of the small things

comm compares two sorted lists in one pass

"Which ids are in file A but not in B?" is a question comm answers directly. It prints three columns: only in A, only in B, and in both.

comm -23 <(sort a.txt) <(sort b.txt)   # only in a
comm -12 <(sort a.txt) <(sort b.txt)   # in both

Both inputs have to be sorted, and the process substitution sorts them inline.

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

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