Add Idempotent Inserts as a Postgres TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1204 TILs and counting..._
+_1205 TILs and counting..._
 
 ---
 
@@ -609,6 +609,7 @@ _1204 TILs and counting..._
 - [Get The Size Of An Index](postgres/get-the-size-of-an-index.md)
 - [Getting A Slice Of An Array](postgres/getting-a-slice-of-an-array.md)
 - [Group By The Result Of A Function Call](postgres/group-by-the-result-of-a-function-call.md)
+- [Idempotent Inserts](postgres/idempotent-inserts.md)
 - [Include All Queries In The Log File](postgres/include-all-queries-in-the-log-file.md)
 - [Insert A Bunch Of Records With Generate Series](postgres/insert-a-bunch-of-records-with-generate-series.md)
 - [Insert Just The Defaults](postgres/insert-just-the-defaults.md)

postgres/idempotent-inserts.md

@@ -0,0 +1,29 @@
+# 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.