Forum OpenACS Q&A: I still don't understand locking in PG

the statement

        db_dml reset_hp_u "update minions set hp = hp_max"

is deadlocking with other updates to minions. I thought simple updates like this are supposed to go lock all the rows they need before proceeding. (Guess not?) Even when I try this

    db_transaction {
        db_dml lock "lock table minions in row share mode"
        db_dml reset_hp_u "update minions set hp = hp_max"
    }

it gets rejected. (row share = others can read but don't let anyone write anything 'til I'm done, right?)