Make compatible for all versions 9.5 to 11.

Supporting 9.4 would add a lot of mess to the code and it's about to be
EOL anyway so don't bother.  A lot has changed for v12 and it's not out
yet, so do that in a separate update.

README.md

@@ -1,5 +1,7 @@
 # A `periods` Extension
 
+*compatible 9.5--11*
+
 This extension attemps to recreate the behavior defined in SQL:2016
 (originally SQL:2011) around periods and tables with `SYSTEM
 VERSIONING`. The idea is to figure out all the rules that PostgreSQL

expected/periods.out

@@ -1,6 +1,6 @@
 /* Install the extension */
-CREATE EXTENSION periods CASCADE;
-NOTICE:  installing required extension "btree_gist"
+CREATE EXTENSION IF NOT EXISTS btree_gist;
+CREATE EXTENSION periods;
 /*
  * Test creating a table, dropping a column, and then dropping the whole thing;
  * without any periods.
@@ -133,29 +133,17 @@ SELECT periods.add_system_time_period('sysver');
  t
 (1 row)
 
-\d+ sysver
-                                                               Table "public.sysver"
-      Column       |           Type           | Collation | Nullable |               Default                | Storage  | Stats target | Description 
--------------------+--------------------------+-----------+----------+--------------------------------------+----------+--------------+-------------
- val               | text                     |           |          |                                      | extended |              | 
- startname         | timestamp with time zone |           | not null | 'infinity'::timestamp with time zone | plain    |              | 
- system_time_end   | timestamp with time zone |           | not null | 'infinity'::timestamp with time zone | plain    |              | 
- system_time_start | timestamp with time zone |           | not null | transaction_timestamp()              | plain    |              | 
- endname           | timestamp with time zone |           | not null | transaction_timestamp()              | plain    |              | 
-Check constraints:
-    "sysver_system_time_check" CHECK (system_time_start < system_time_end)
-    "sysver_system_time_end_infinity_check" CHECK (system_time_end = 'infinity'::timestamp with time zone)
-Triggers:
-    sysver_system_time_generated_always BEFORE INSERT OR UPDATE ON sysver FOR EACH ROW EXECUTE PROCEDURE periods.generated_always_as_row_start_end()
-    sysver_system_time_write_history AFTER INSERT OR DELETE OR UPDATE ON sysver FOR EACH ROW EXECUTE PROCEDURE periods.write_history()
-    sysver_truncate AFTER TRUNCATE ON sysver FOR EACH STATEMENT EXECUTE PROCEDURE periods.truncate_system_versioning()
-
 DROP TABLE sysver;
 TABLE periods.periods;
  table_name | period_name | start_column_name | end_column_name | range_type | bounds_check_constraint 
 ------------+-------------+-------------------+-----------------+------------+-------------------------
 (0 rows)
 
+TABLE periods.system_time_periods;
+ table_name | period_name | infinity_check_constraint | generated_always_trigger | write_history_trigger | truncate_trigger 
+------------+-------------+---------------------------+--------------------------+-----------------------+------------------
+(0 rows)
+
 /* Basic SYSTEM VERSIONING */
 CREATE TABLE sysver (val text);
 SELECT periods.add_system_time_period('sysver');
@@ -343,10 +331,9 @@ TABLE periods.periods;
 ------------+-------------+-------------------+-----------------+------------+-------------------------
 (0 rows)
 
-\d
-      List of relations
- Schema | Name | Type | Owner 
---------+------+------+-------
+TABLE periods.system_time_periods;
+ table_name | period_name | infinity_check_constraint | generated_always_trigger | write_history_trigger | truncate_trigger 
+------------+-------------+---------------------------+--------------------------+-----------------------+------------------
 (0 rows)
 
 /* Clean up */

periods.c

@@ -16,6 +16,10 @@ PG_MODULE_MAGIC;
 PG_FUNCTION_INFO_V1(generated_always_as_row_start_end);
 PG_FUNCTION_INFO_V1(write_history);
 
+/* Define some SQLSTATEs that might not exist */
+#if (PG_VERSION_NUM < 100000)
+#define ERRCODE_GENERATED_ALWAYS MAKE_SQLSTATE('4','2','8','C','9')
+#endif
 #define ERRCODE_INVALID_ROW_VERSION MAKE_SQLSTATE('2','2','0','1','H')
 
 void GetPeriodColumnNames(Relation rel, char *period_name, char **start_name, char **end_name);
@@ -183,7 +187,11 @@ generated_always_as_row_start_end(PG_FUNCTION_ARGS)
 	columns[1] = end_num;
 	values[1] = TimestampTzGetDatum(DT_NOEND);
 	nulls[1] = false;
+#if (PG_VERSION_NUM < 100000)
+	new_row = SPI_modifytuple(rel, new_row, 2, columns, values, nulls);
+#else
 	new_row = heap_modify_tuple_by_cols(new_row, new_tupdesc, 2, columns, values, nulls);
+#endif
 
 	return PointerGetDatum(new_row);
 }

sql/periods.sql

@@ -1,5 +1,6 @@
 /* Install the extension */
-CREATE EXTENSION periods CASCADE;
+CREATE EXTENSION IF NOT EXISTS btree_gist;
+CREATE EXTENSION periods;
 
 /*
  * Test creating a table, dropping a column, and then dropping the whole thing;
@@ -44,9 +45,9 @@ SELECT periods.drop_system_time_period('sysver', drop_behavior => 'CASCADE', pur
 SELECT periods.add_system_time_period('sysver', 'endname', 'startname');
 SELECT periods.drop_system_time_period('sysver', drop_behavior => 'CASCADE', purge => true);
 SELECT periods.add_system_time_period('sysver');
-\d+ sysver
 DROP TABLE sysver;
 TABLE periods.periods;
+TABLE periods.system_time_periods;
 
 /* Basic SYSTEM VERSIONING */
 
@@ -116,7 +117,7 @@ SELECT periods.drop_system_versioning('sysver', drop_behavior => 'CASCADE', purg
 TABLE periods.system_versioning;
 DROP TABLE sysver;
 TABLE periods.periods;
-\d
+TABLE periods.system_time_periods;
 
 
 /* Clean up */