Add Idempotent Inserts as a Postgres TIL

postgres/idempotent-inserts.md

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+# Idempotent Inserts
+
+I'm writing a bunch of records from one table to another using
+[`insert`](https://www.postgresql.org/docs/current/sql-insert.html). This is
+part of a process to swap out a massive table with an identical version that
+has a fraction of the records.
+
+I want to copy a subset of rows from the primary table to the replacement table
+that meet certain criteria. This query should be re-runnable and idempotent.
+That way I can run it, investage the replacement table, and then run it again
+before swapping out the tables. This helps ensure I don't miss anything.
+
+Such an idempotent query can be written with the help of the `on conflict`
+clause.
+
+```sql
+insert into replacement_table
+select * from original_table
+where created_at > (now() - '3 months'::interval)
+on conflict do nothing;
+```
+
+If I run that multiple times, it skips over any records that would otherwise
+cause a conflict. The unique primary key column is usually what determines a
+conflicting record.
+
+Without the `on conflict do nothing`, trying to run this `insert` multiple
+times will fail on subsequent runs when it tries to re-insert rows with primary
+keys it has already inserted.