Hello, I have an query with the same source data on Jan2014 -SP3 version of monetddb and Oct2014 - SP1 : CREATE TABLE tmp __pierre5 AS ( SELECT id_unique, Device , COUNT (*) AS NBVISISTES FROM donnees_lmd_pierre GROUP BY id_unique, Device ) WITH DATA; On version Jan2014 -SP3 : Welcome to mclient , the MonetDB / SQL interactive terminal (Jan2014-SP3) Database : MonetDB v11.17.21 (Jan2014-SP3), 'mapi : monetdb :// lemondedev :50000/ lemonde' Type \q to quit , \? for a list of available commands auto commit mode: on sql > CREATE TABLE tmp __pierre5 AS ( SELECT id_unique, Device , COUNT (*) AS NBVISISTES FROM donnees _ lmd _pierre GROUP BY id_unique, Device ) WITH DATA; operation successful (6m 30s) On version Oct2014-SP1 : Welcome to mclient , the MonetDB / SQL interactive terminal (Oct2014-SP1) Database : MonetDB v11.19.7 (Oct2014-SP1), 'mapi : monetdb :// monetdb :50000/ lemonde' Type \q to quit , \? for a list of available commands auto commit mode: on sql > CREATE TABLE tmp __pierre5 AS ( SELECT id_unique, Device , COUNT (*) AS NBVISISTES FROM donnees _ lmd _pierre GROUP BY id_unique, Device ) WITH DATA; operation successful (383m 20s) explain query is different , you want to explain the results of the query ? Pierre -- 1G6 52 route de bischwiller 67300 Schiltigheim Société de Services et de Formations en Logiciels Libres http://1g6.biz Tél : 06 64 63 70 35