Add Use A Trigger To Mirror Inserts To Another Table as a Postgres TIL

postgres/use-a-trigger-to-mirror-inserts-to-another-table.md

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+# Use A Trigger To Mirror Inserts To Another Table
+
+On a PostgreSQL server, a trigger can be set up to execute a function whenever
+a certain action happens. In this case, I want set up a trigger to call a
+custom function whenever an `insert` happens on a specific table
+(`original_table`). That custom function will then mirror the inserted values
+into a secondary table (`another_table`).
+
+First, I have to create a function that will respond to `insert` operations by
+inserting the newly inserted rows into `another_table`.
+
+```sql
+create or replace function mirror_table_to_another_table()
+  returns trigger as $mirrored_table$
+    begin
+      if (TG_OP = 'insert') then
+        insert into another_table
+          select * from new_table;
+      end if;
+      return null; -- result is ignored since this is an after trigger
+    end;
+$mirrored_table$ language plpgsql;
+```
+
+This function can then be referenced by the trigger I set up. After any insert
+on the `original_table`, the function defined above will be executed.
+
+```sql
+create trigger mirror_table_to_another_table_trigger
+  after insert on original_table
+  referencing new table as new_table
+  for each statement
+    execute function mirror_table_to_another_table();
+```
+
+Note that I am handling inserts at a statement level and that multiple rows can
+be inserted in a single statement. That is why the function mirrors to the
+other table with `select * from new_table`.