Add Generate Random Alphanumeric Identifier as a Postgres TIL

postgres/generate-random-alphanumeric-identifier.md

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+# Generate Random Alphanumeric Identifier
+
+Here is a PostgreSQL query that uses
+[`pgcrypto`](https://www.postgresql.org/docs/current/pgcrypto.html) (for
+[`get_random_bytes`](https://www.postgresql.org/docs/current/pgcrypto.html#PGCRYPTO-RANDOM-DATA-FUNCS))
+and a CTE to generate a cryptographically-random 8-character alphanumeric
+identifier.
+
+```sql
+-- First ensure pgcrypto is installed
+create extension if not exists pgcrypto;
+
+-- Generates a single 8-character identifier
+with chars as (
+  -- excludes some look-alike characters
+  select '23456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnpqrstuvwxyz' as charset
+),
+random_bytes as (
+  select gen_random_bytes(8) as bytes
+),
+positions as (
+  select generate_series(0, 7) as pos
+)
+select string_agg(
+  substr(
+    charset,
+    (get_byte(bytes, pos) % length(charset)) + 1,
+    1
+  ),
+  '' order by pos
+) as short_id
+from positions, random_bytes, chars;
+```
+
+The
+[`generate_series`](https://www.postgresql.org/docs/current/functions-srf.html)
+gives us an 8-row table from 0 to 7 that we can use as indexes into the byte
+positions of the value we get from `gen_random_bytes`. Those random bytes get
+mapped to individual alphanumeric characters from `chars`. That then gets
+squeezed together with `string_agg`.
+
+Note: the character set excludes some characters that can be mistaken for one
+another like `0` and `O` or `1` and `l`.
+
+Note: you could change the right-bound of the `generate_series` to generate a
+different length identifier.