Try new operation table

script/sql/api/1.title.sql

@@ -0,0 +1,37 @@
+drop function if exists get_title;
+
+create function get_title() returns text as
+$$
+select value from title as result;
+$$
+language sql;
+
+
+drop function if exists set_title;
+
+create function set_title(new_value text) returns json as
+$$
+declare
+    old_value text;
+
+    redo_sql text;
+    undo_sql text;
+    redo_cs json;
+    undo_cs json;
+
+begin
+    select value into old_value from title;
+
+    redo_sql = format('update title set value = ''%s''', new_value);
+    undo_sql = format('update title set value = ''%s''', old_value);
+    redo_cs = format('{"operation": "update", "type": "title", "value": "%s" }', new_value)::json;
+    undo_cs = format('{"operation": "update", "type": "title", "value": "%s" }', old_value)::json;
+    execute add_operation(redo_sql, undo_sql, redo_cs, undo_cs);
+
+    execute redo_sql;
+
+    return redo_cs;
+    
+end;
+$$
+language plpgsql;

script/sql/api/operation.sql

@@ -0,0 +1,85 @@
+drop function if exists get_current_operation;
+
+create function get_current_operation() returns integer as
+$$
+select id from current_operation as result;
+$$
+language sql;
+
+
+drop function if exists add_operation;
+
+create function add_operation(redo_sql text, undo_sql text, redo_cs json, undo_cs json) returns void as
+$$
+declare
+    parent_id integer;
+    op_id integer;
+
+begin
+    parent_id := get_current_operation();
+
+    insert into operation (id, redo, undo, parent, redo_change_set, undo_change_set)
+        values (default, redo_sql, undo_sql, parent_id, redo_cs, undo_cs);
+
+    select max(id) into op_id from operation;
+
+    update current_operation set id = op_id where id = parent_id;
+
+end;
+$$
+language plpgsql;
+
+
+drop function if exists execute_undo;
+
+create function execute_undo(discard boolean) returns json as
+$$
+#print_strict_params on
+
+declare
+    op_row operation%ROWTYPE;
+
+begin
+    select * into strict op_row from operation where id = get_current_operation();
+    if op_row.undo = '' then
+        return '{}'::json;
+    end if;
+
+    execute op_row.undo;
+
+    -- update foreign key
+    update current_operation set id = op_row.parent where id = op_row.id;
+
+    if discard then
+        -- update foreign key
+        update operation set redo_child = null where id = op_row.parent;
+        -- on delete cascade => child & snapshot
+        delete from operation where id = op_row.id;
+    else
+        update operation set redo_child = op_row.id where id = op_row.parent;
+    end if;
+
+    return op_row.undo_change_set;
+
+end;
+$$
+language plpgsql;
+
+
+-- create function execute_redo() returns json as
+-- $$
+-- #print_strict_params on
+-- 
+-- declare
+--     op_id integer;
+--     redo_c integer;
+--     redo_row operation%ROWTYPE;
+-- 
+-- begin
+--     op_id := get_current_operation();
+-- 
+--     select redo_child into redo_c from operation where id = op_id;
+-- 
+-- end;
+-- $$
+-- language plpgsql;