Add Truncate Tables With Dependants as a postgres til.

README.md

@@ -130,6 +130,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Timestamp Functions](postgres/timestamp-functions.md)
 - [Toggling The Pager In PSQL](postgres/toggling-the-pager-in-psql.md)
 - [Truncate All Rows](postgres/truncate-all-rows.md)
+- [Truncate Tables With Dependants](postgres/truncate-tables-with-dependants.md)
 - [Turning Timing On](postgres/turning-timing-on.md)
 - [Types By Category](postgres/types-by-category.md)
 - [Use Argument Indexes](postgres/use-argument-indexes.md)

postgres/truncate-tables-with-dependants.md

@@ -0,0 +1,37 @@
+# Truncate Tables With Dependants
+
+In [Truncate All Rows](postgres/truncate-all-rows.md), I talked about how
+postgres's `truncate` can be used to quickly delete all rows in a table. In
+practice this alone won't be very useful though, because tables usually have
+other tables that depend on them via foreign keys. If you have tables `A`
+and `B` where `B` has a foreign key referencing `A`, then trying to truncate
+`A` will result in something like this:
+
+```sql
+> truncate A;
+ERROR:  cannot truncate a table referenced in a foreign key constraint
+```
+
+Fortunately, `truncate` has some tricks up its sleeve.
+
+If you know two tables are tied together via a foreign key constraint, you
+can just truncate both of them at once:
+
+```sql
+> truncate A, B;
+TRUNCATE TABLE;
+```
+
+If many tables are tied together in this way and you are looking to throw
+all of it out, then a simpler approach is to cascade the truncation:
+
+```
+> truncate A cascade;
+NOTICE:  truncate cascades to table "B"
+TRUNCATE TABLE
+```
+
+Use these with care and potentially within transactions because your data
+will go bye bye.
+
+h/t Dillon Hafer and Jack Christensen