equivalent in postgres of over(partition) function in db2

Hi,

I have the following query in db2:

"insert into xyz (a,b) select 10, MIN(CM_LEDGERITEM.tril_gid) OVER(PARTITION
BY CM_LEDGERITEM.MMPAYEE, CM_PROMOTION.MMCOMPTYPE, CM_LEDGERITEM.MODEL) from
...."

I am not able to find a corresponding thing for min(...) over(partition by
col1, col2, col3..)
in postgres.What will be its equivalent in postgres??

Regards,
Vinita Bansal

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