diff --git a/sql/first_last_agg--0.1.4--0.1.5.sql b/sql/first_last_agg--0.1.4--0.1.5.sql
index 82d135b..94be866 100644
--- a/sql/first_last_agg--0.1.4--0.1.5.sql
+++ b/sql/first_last_agg--0.1.4--0.1.5.sql
@@ -1,28 +1,51 @@
 /*
- * Make aggregate functions parallel safe, starting with PG10.
+ * Upgrade from 0.1.4 to 0.1.5: make aggregate functions parallel safe on
+ * supported server versions (PostgreSQL 12+).
+ *
+ * NOTE: upgrade limitations
+ *
+ * This upgrade requires PostgreSQL 12 or later. CREATE OR REPLACE AGGREGATE,
+ * which is needed to safely redefine aggregates that may have dependent
+ * objects, was introduced in PostgreSQL 12. Attempting this upgrade on an
+ * older version will raise an error; stay on 0.1.4 in that case.
+ *
+ * NOTE: parallel safety and sorted aggregates
+ *
+ * The primary use of first()/last() is with per-aggregate ORDER BY, such as
+ * "last(col ORDER BY sort_col)".
+ * As of PostgreSQL 18, PostgreSQL does not apply the Parallel Aggregate plan
+ * node to aggregates with ORDER BY or DISTINCT modifiers, so PARALLEL SAFE +
+ * COMBINEFUNC have no effect on query performance for that pattern.
+ * ref. https://www.postgresql.org/docs/18/parallel-plans.html#PARALLEL-AGGREGATION
+ *
+ * Parallel aggregation is currently only expected to apply outside of these
+ * cases.
  */
+
 DO $$
-DECLARE version_num integer;
 BEGIN
-  SELECT current_setting('server_version_num') INTO STRICT version_num;
-  IF version_num > 90600 THEN
-    EXECUTE $E$ ALTER FUNCTION last_sfunc(anyelement, anyelement) PARALLEL SAFE   $E$;
-    EXECUTE $E$ ALTER FUNCTION first_sfunc(anyelement, anyelement) PARALLEL SAFE   $E$;
-
-    EXECUTE $E$ CREATE OR REPLACE AGGREGATE first(anyelement) (
-        SFUNC = first_sfunc,
-        STYPE = anyelement,
-        COMBINEFUNC = first_sfunc,
-        parallel = SAFE
-    ); $E$;
-
-    EXECUTE $E$ CREATE OR REPLACE AGGREGATE last(anyelement) (
-        SFUNC = last_sfunc,
-        STYPE = anyelement,
-        COMBINEFUNC = last_sfunc,
-        parallel = SAFE
-    ); $E$;
+  IF current_setting('server_version_num')::integer < 120000 THEN
+    RAISE EXCEPTION
+      'Upgrading first_last_agg to 0.1.5 requires PostgreSQL 12 or later '
+      '(current version: %). Stay on version 0.1.4 or upgrade your PostgreSQL.',
+      current_setting('server_version');
   END IF;
 END;
 $$;
 
+ALTER FUNCTION last_sfunc(anyelement, anyelement) PARALLEL SAFE;
+ALTER FUNCTION first_sfunc(anyelement, anyelement) PARALLEL SAFE;
+
+CREATE OR REPLACE AGGREGATE first(anyelement) (
+    SFUNC = first_sfunc,
+    STYPE = anyelement,
+    COMBINEFUNC = first_sfunc,
+    PARALLEL = SAFE
+);
+
+CREATE OR REPLACE AGGREGATE last(anyelement) (
+    SFUNC = last_sfunc,
+    STYPE = anyelement,
+    COMBINEFUNC = last_sfunc,
+    PARALLEL = SAFE
+);
diff --git a/sql/first_last_agg--0.1.4--0.1.6.sql b/sql/first_last_agg--0.1.4--0.1.6.sql
deleted file mode 100644
index 75a332e..0000000
--- a/sql/first_last_agg--0.1.4--0.1.6.sql
+++ /dev/null
@@ -1,29 +0,0 @@
-/*
- * Make aggregate functions parallel safe for supported server versions.
- */
-DO $$
-DECLARE version_num integer;
-BEGIN
-  SELECT current_setting('server_version_num') INTO STRICT version_num;
-  IF version_num > 90600 THEN
-    EXECUTE $E$ ALTER FUNCTION last_sfunc(anyelement, anyelement) PARALLEL SAFE $E$;
-    EXECUTE $E$ ALTER FUNCTION first_sfunc(anyelement, anyelement) PARALLEL SAFE $E$;
-
-    EXECUTE $E$ DROP AGGREGATE IF EXISTS first(anyelement) $E$;
-    EXECUTE $E$ CREATE AGGREGATE first(anyelement) (
-        SFUNC = first_sfunc,
-        STYPE = anyelement,
-        COMBINEFUNC = first_sfunc,
-        PARALLEL = SAFE
-    ) $E$;
-
-    EXECUTE $E$ DROP AGGREGATE IF EXISTS last(anyelement) $E$;
-    EXECUTE $E$ CREATE AGGREGATE last(anyelement) (
-        SFUNC = last_sfunc,
-        STYPE = anyelement,
-        COMBINEFUNC = last_sfunc,
-        PARALLEL = SAFE
-    ) $E$;
-  END IF;
-END;
-$$;
diff --git a/sql/first_last_agg--0.1.5--0.1.4.sql b/sql/first_last_agg--0.1.5--0.1.4.sql
index fe4578b..b75752e 100644
--- a/sql/first_last_agg--0.1.5--0.1.4.sql
+++ b/sql/first_last_agg--0.1.5--0.1.4.sql
@@ -1,24 +1,41 @@
 /*
- * Downgrade and create functions and aggregates without parallel safe.
+ * Downgrade from 0.1.5 to 0.1.4: remove parallel safety from aggregate
+ * functions.
+ *
+ * NOTE: downgrade limitations
+ *
+ * This downgrade requires PostgreSQL 12 or later, for the same reason as
+ * the upgrade: CREATE OR REPLACE AGGREGATE (needed to safely redefine
+ * aggregates with potential dependent objects) was introduced in PostgreSQL
+ * 12.
+ * Databases below PG 12 may have reached 0.1.5+ via a fresh installation
+ * or the old direct upgrade path (which used DROP+CREATE) if they had no
+ * dependency.  Those databases cannot use this downgrade script.
+ * Since PG 10 and PG 11 are both EOL, no workaround is provided.  If you face
+ * this problem, you will have to upgrade your PostgreSQL instance to at least
+ * PostgreSQL 12 in order to be able to downgrade the extension.
  */
+
 DO $$
-DECLARE version_num integer;
 BEGIN
-  SELECT current_setting('server_version_num') INTO STRICT version_num;
-  IF version_num > 90600 THEN
-    EXECUTE $E$ ALTER FUNCTION last_sfunc(anyelement, anyelement) PARALLEL UNSAFE   $E$;
-    EXECUTE $E$ ALTER FUNCTION first_sfunc(anyelement, anyelement) PARALLEL UNSAFE   $E$;
-
-    EXECUTE $E$ CREATE OR REPLACE AGGREGATE first(anyelement) (
-        SFUNC = first_sfunc,
-        STYPE = anyelement
-    ); $E$;
-
-    EXECUTE $E$ CREATE OR REPLACE AGGREGATE last(anyelement) (
-        SFUNC = last_sfunc,
-        STYPE = anyelement
-    ); $E$;
+  IF current_setting('server_version_num')::integer < 120000 THEN
+    RAISE EXCEPTION
+      'Downgrading first_last_agg from 0.1.5 requires PostgreSQL 12 or later '
+      '(current version: %).',
+      current_setting('server_version');
   END IF;
 END;
 $$;
 
+ALTER FUNCTION last_sfunc(anyelement, anyelement) PARALLEL UNSAFE;
+ALTER FUNCTION first_sfunc(anyelement, anyelement) PARALLEL UNSAFE;
+
+CREATE OR REPLACE AGGREGATE first(anyelement) (
+    SFUNC = first_sfunc,
+    STYPE = anyelement
+);
+
+CREATE OR REPLACE AGGREGATE last(anyelement) (
+    SFUNC = last_sfunc,
+    STYPE = anyelement
+);
diff --git a/sql/first_last_agg--0.1.5--0.1.6.sql b/sql/first_last_agg--0.1.5--0.1.6.sql
index 4bea590..54978a8 100644
--- a/sql/first_last_agg--0.1.5--0.1.6.sql
+++ b/sql/first_last_agg--0.1.5--0.1.6.sql
@@ -1,4 +1,17 @@
 /*
- * Version 0.1.6 keeps the SQL object definitions from 0.1.5 unchanged.
- * This release only adds a PostgreSQL 10-safe direct migration path from 0.1.4.
+ * No-op upgrade from 0.1.5 to 0.1.6.
+ *
+ * Version 0.1.6 introduces no changes to SQL object definitions.
+ *
+ * History of this upgrade path:
+ * - 0.1.4 defined the aggregates without parallel support.
+ * - 0.1.5 added PARALLEL SAFE + COMBINEFUNC, requiring PostgreSQL 12+.
+ * - 0.1.6 was originally intended to provide a PostgreSQL 10-safe direct:
+ *   - migration path from 0.1.4 via DROP + CREATE with a dependency check
+ *   - that approach has been superseded: 0.1.5+ now simply requires PG 12+
+ *     and uses CREATE OR REPLACE AGGREGATE
+ *   - the SQL object definitions in 0.1.5 and 0.1.6 are therefore identical
+ *
+ * Databases already at 0.1.5 already have the parallel-safe definitions, so
+ * nothing needs to be done here.
  */
diff --git a/sql/first_last_agg--0.1.6--0.1.5.sql b/sql/first_last_agg--0.1.6--0.1.5.sql
index 032ddac..4574ced 100644
--- a/sql/first_last_agg--0.1.6--0.1.5.sql
+++ b/sql/first_last_agg--0.1.6--0.1.5.sql
@@ -1,4 +1,7 @@
 /*
- * Version 0.1.6 keeps the SQL object definitions from 0.1.5 unchanged.
- * This downgrade exists to preserve an explicit path between the tagged releases.
+ * No-op downgrade from 0.1.6 to 0.1.5.
+ *
+ * Version 0.1.6 and 0.1.5 have identical SQL object definitions, so there is
+ * nothing to revert.  See first_last_agg--0.1.5--0.1.6.sql for the version
+ * history explaining why.
  */
diff --git a/sql/first_last_agg.sql b/sql/first_last_agg.sql
index 3978568..b4c282e 100644
--- a/sql/first_last_agg.sql
+++ b/sql/first_last_agg.sql
@@ -20,6 +20,20 @@ CREATE AGGREGATE last(anyelement) (
     STYPE = anyelement
 );
 
+/*
+ * NOTE: parallel safety and sorted aggregates
+ *
+ * The primary use of first()/last() is with per-aggregate ORDER BY, such as
+ * "last(col ORDER BY sort_col)".
+ * As of PostgreSQL 18, PostgreSQL does not apply the Parallel Aggregate plan
+ * node to aggregates with ORDER BY or DISTINCT modifiers, so PARALLEL SAFE +
+ * COMBINEFUNC have no effect on query performance for that pattern.
+ * ref.
+ * https://www.postgresql.org/docs/18/parallel-plans.html#PARALLEL-AGGREGATION
+ *
+ * So parallel aggregation is currently only expected to apply outside of these
+ * cases.
+ */
 
 DO $$
 DECLARE version_num integer;
@@ -46,4 +60,4 @@ BEGIN
     ); $E$;
   END IF;
 END;
-$$;
\ No newline at end of file
+$$;