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

Author: jbranchaud

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).
 
-_1330 TILs and counting..._
+_1331 TILs and counting..._
 
 ---
 
@@ -740,6 +740,7 @@ _1330 TILs and counting..._
 - [Types By Category](postgres/types-by-category.md)
 - [Union All Rows Including Duplicates](postgres/union-all-rows-including-duplicates.md)
 - [Use A psqlrc File For Common Settings](postgres/use-a-psqlrc-file-for-common-settings.md)
+- [Use A Trigger To Mirror Inserts To Another Table](postgres/use-a-trigger-to-mirror-inserts-to-another-table.md)
 - [Use Argument Indexes](postgres/use-argument-indexes.md)
 - [Use Not Valid To Immediately Enforce A Constraint](postgres/use-not-valid-to-immediately-enforce-a-constraint.md)
 - [Use Rename To Hot Swap Two Tables](postgres/use-rename-to-hot-swap-two-tables.md)

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

@@ -0,0 +1,38 @@
+# 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`.