The initPlan has been moved from the Result node to the Gather node. As a result, when doing tuple projection for the Result node, we'd get a ParamExecData entry with NULL execPlan. So the initPlan does not get chance to be executed. And we'd get the output as the default value from the ParamExecData entry, which is zero as shown.
So now I begin to wonder if this wrong result issue is possible to exist in other places where we move initPlans. But I haven't tried hard to verify that.
I looked further into this issue and I believe other places are good. The problem with this query is that the es/ecxt_param_exec_vals used to store info about the initplan is not the same one as in the Result node's expression context for projection, because we've forked a new process for the parallel worker and then created and initialized a new EState node, and allocated a new es_param_exec_vals array for the new EState. When doing projection for the Result node, the current code just goes ahead and accesses the new es_param_exec_vals, thus fails to retrieve the info about the initplan. Hmm, I doubt this is sensible.
So now it seems that the breakage of regression tests is more severe than being cosmetic. I wonder if we need to update the comments to indicate the potential wrong results issue if we move the initPlans to the Gather node.
