Almost there. As suggested by Fabian in the following thread I gave up the MX file approach and implemented group_concat as UDF in C. http://sourceforge.net/mailarchive/forum.php?thread_name=mailman.243.1355328304.1624.developers-list%40monetdb.org&forum_name=monetdb-developers When I try to conclude my function UDFBATgroup_concat_(BAT **ret, BAT *b, BAT *g, BAT *h, FILE* fp) //ignore fp as follows: BATaccessBegin(h, USE_HEAD | USE_TAIL, MMAP_SEQUENTIAL); index = 0; BATloop(h, p, q) { ptr hhead = BUNhead(hi, p); fprintf(fp, "%d: hhead=%d", index, hhead); str dst = tailValArray[index]; fprintf(fp, "%d: dstStr=%s# ", index, dst); BUNins(bn, hhead, dst, FALSE); fprintf(fp, "%d: Insert complete\n", index); index++; } I get the following error on the client terminal [tapomay@tapomay-laptop monetdata]$ echo "select house, group_concat(name) from gs group by house;" | mclient -d tapomay GDKerror:!ERROR: BUNins: access denied to tmp_10162, aborting. BUNins: access denied to tmp_10162, aborting. BUNins: access denied to tmp_10162, aborting. The groupings are ready in the tailVarArray. NEED HELP. Thanks and Regards, Tapomay. _______________________________________________ developers-list mailing list developers-list@monetdb.org http://mail.monetdb.org/mailman/listinfo/developers-list