refact(db): Implement target history base and subtype table design

internal/db/schema/migrations/oss/postgres/99902/01_target_history_base.up.sql

@@ -0,0 +1,63 @@
+-- Copyright (c) HashiCorp, Inc.
+-- SPDX-License-Identifier: BUSL-1.1
+
+-- This migration refactors target history to a base + subtype table design,
+-- like we have for hosts. This design has a target_history_base table (the base
+-- table) which holds all history ids for all target subtypes that have history
+-- subtype tables. Data is inserted into or deleted from this base table via
+-- triggers on each subtype table.
+begin;
+  create table target_history_base(
+    history_id wt_url_safe_id primary key
+  );
+  comment on table target_history_base is
+    'target_history_base is a base history table '
+    'for target subtype history tables.';
+
+  create function insert_target_history_subtype() returns trigger
+  as $$
+  begin
+    insert into target_history_base
+      (history_id)
+    values
+      (new.history_id);
+    return new;
+  end;
+  $$ language plpgsql;
+  comment on function insert_target_history_subtype() is
+    'insert_target_history_subtype() is a before insert trigger'
+    'function for subtypes of target_history_base';
+
+  create function delete_target_history_subtype() returns trigger
+  as $$
+  begin
+    delete
+      from target_history_base
+      where history_id = old.history_id;
+    return null;
+  end;
+  $$ language plpgsql;
+  comment on function delete_target_history_subtype() is
+    'delete_target_history_subtype() is an after delete trigger'
+    'function for subtypes of target_history_base';
+
+  -- Update existing SSH target history table to follow the new pattern.
+  -- First, backfill target_history_base with existing SSH history.
+  insert into target_history_base (history_id)
+    select history_id from target_ssh_hst;
+
+  -- Next, set target_ssh_hst's history_id field to be a FK of the base table.
+  alter table target_ssh_hst
+    add constraint target_history_base_fkey
+      foreign key (history_id)
+        references target_history_base (history_id)
+        on delete cascade
+        on update cascade;
+
+  -- Finally, add new triggers to target_ssh_hst to automatically insert into
+  -- target_history_base from now on.
+  create trigger hst_before_insert before insert on target_ssh_hst
+    for each row execute function insert_target_history_subtype();
+  create trigger hst_after_delete after delete on target_ssh_hst
+    for each row execute function delete_target_history_subtype();
+commit;