Add ENV["SKIP_TEST_DATABASE_TRUNCATE"] flag to speed up multi-process test runs (rails#51686)

Author: dhh

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add ENV["SKIP_TEST_DATABASE_TRUNCATE"] flag to speed up multi-process test runs on large DBs when all tests run within default txn. (This cuts ~10s from the test run of HEY when run by 24 processes against the 178 tables, since ~4,000 table truncates can then be skipped.)
+
+    *DHH*
+
 *   Added support for recursive common table expressions.
 
     ```ruby

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -390,7 +390,7 @@ def reconstruct_from_schema(db_config, format = ActiveRecord.schema_format, file
 
         with_temporary_pool(db_config, clobber: true) do
           if schema_up_to_date?(db_config, format, file)
-            truncate_tables(db_config)
+            truncate_tables(db_config) unless ENV["SKIP_TEST_DATABASE_TRUNCATE"]
           else
             purge(db_config)
             load_schema(db_config, format, file)