diff --git a/sql/historization.sql b/sql/historization.sql
index 37c266e1..1bf0a165 100644
--- a/sql/historization.sql
+++ b/sql/historization.sql
@@ -21,16 +21,26 @@ create or replace function tx_history_txid()
     language plpgsql as $$
 declare
     historicalTxIdSetting text;
+    historicalTimestampSetting text;
     historicalTxId xid8;
     historicalTimestamp timestamp;
 begin
-    select current_setting('hsadminng.tx_history_txid', true) into historicalTxIdSetting;
-    raise notice 'tx_history_txid(): historicalTxIdSetting=%', historicalTxIdSetting;
+    select coalesce(current_setting('hsadminng.tx_history_txid', true), '') into historicalTxIdSetting;
+    select coalesce(current_setting('hsadminng.tx_history_timestamp', true), '') into historicalTimestampSetting;
+    if historicalTxIdSetting > '' and historicalTimestampSetting > '' then
+        raise exception 'either hsadminng.tx_history_txid or hsadminng.tx_history_timestamp must be set, but both are set: (%, %)',
+            historicalTxIdSetting, historicalTimestampSetting;
+    end if;
+    if historicalTxIdSetting = '' and historicalTimestampSetting = '' then
+        raise exception 'either hsadminng.tx_history_txid or hsadminng.tx_history_timestamp must be set, but both are unset or empty: (%, %)',
+            historicalTxIdSetting, historicalTimestampSetting;
+    end if;
+    -- just for debugging / making sure the function is only called once per query
+    -- raise notice 'tx_history_txid() called with: (%, %)', historicalTxIdSetting, historicalTimestampSetting;
+
     if historicalTxIdSetting is null or historicalTxIdSetting = '' then
-        select current_setting('hsadminng.tx_history_timestamp', true)::timestamp into historicalTimestamp;
-        raise notice 'tx_history_txid(): historicalTimestamp=%', historicalTimestamp;
+        select historicalTimestampSetting::timestamp into historicalTimestamp;
         select max(txc.txid) from tx_context txc where txc.txtimestamp <= historicalTimestamp into historicalTxId;
-        raise notice 'tx_history_txid(): historicalTxId=%', historicalTxId;
     else
         historicalTxId = historicalTxIdSetting::xid8;
     end if;
@@ -169,7 +179,7 @@ commit;
 -- single version at point in time
 -- set hsadminng.tx_history_txid to (select max(txid) from tx_context where txtimestamp<='2024-08-27 12:13:13.450821');
 set hsadminng.tx_history_txid to '3249';
---set hsadminng.tx_history_timestamp to '2024-08-27 17:52:07.755407';
+set hsadminng.tx_history_timestamp to '';
 -- all versions
 select tx_history_txid(), txc.txtimestamp, txc.currentUser, txc.currentTask, haex.*
     from hs_hosting_asset_ex haex