Hello,

The bug can be reproduced on a standard TPC-H database with scale factor 1.  A query returns NULL after query logging is enabled.  The computer we use has Intel i7-8700K (12 cores) and 32GB RAM.

Please see the details below. 

>> mclient -d tpch1 -t performance
Welcome to mclient, the MonetDB/SQL interactive terminal (Nov2019-SP1)
Database: MonetDB v11.35.9 (Nov2019-SP1), 'mapi:monetdb://tigger:50000/tpch1'
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sql>select sum(l_extendedprice) from lineitem;
+------------------------------------------+
| L2                                       |
+==========================================+
|                          229577310901.20 |
+------------------------------------------+
1 tuple
sql:0.083 opt:1.056 run:5.898 clk:7.763 ms
sql>call sys.querylog_enable();
sql>select sum(l_extendedprice) from lineitem;
+------------------------------------------+
| L2                                       |
+==========================================+
|                                     null |
+------------------------------------------+
1 tuple
sql:0.095 opt:1.312 run:15.920 clk:25.033 ms

Hanfeng Chen