Обсуждение: Scans are offloaded to SeqScan instead of CustomScan when there are multiple relations in the same query
Hi there,
In my implementation of CustomScan, when I have a query that scans multiple tables (select c from t1,t2,t3), the planner always picks one table to be scanned by CustomScan and offloads the rest to SeqScan. I tried assigning a cost of 0 to the CustomScan path, but still not working. BeginCustomScan gets executed, ExecCustomScan is skipped, and then EndCustomScan is executed for all the tables that are offloaded to Seq Scan. EXPLAIN shows that always only one table is picked to be executed by CustomScan. Any idea what I might be doing wrong? Like a value in a struct I might be setting incorrectly?
Thanks!
To simplify: Can CustomScan scan multiple relations in the same query or it will always be assigned to one or zero relations?
On Fri, Apr 14, 2023 at 4:33 PM Amin <amin.fallahi@gmail.com> wrote:
Hi there,In my implementation of CustomScan, when I have a query that scans multiple tables (select c from t1,t2,t3), the planner always picks one table to be scanned by CustomScan and offloads the rest to SeqScan. I tried assigning a cost of 0 to the CustomScan path, but still not working. BeginCustomScan gets executed, ExecCustomScan is skipped, and then EndCustomScan is executed for all the tables that are offloaded to Seq Scan. EXPLAIN shows that always only one table is picked to be executed by CustomScan. Any idea what I might be doing wrong? Like a value in a struct I might be setting incorrectly?Thanks!
Amin <amin.fallahi@gmail.com> writes:
> To simplify: Can CustomScan scan multiple relations in the same query or it
> will always be assigned to one or zero relations?
There's barely any code in the core planner that is specific to custom
scans. Almost certainly this misbehavior is the fault of your
custom-path-creation code. Maybe you're labeling the paths with the
wrong parent relation, or forgetting to submit them to add_path,
or assigning them costs that are high enough to get them rejected?
regards, tom lane
Hi Tom,
I made sure EXPLAIN returns CustomScan for all scans in the query. But still, ExecCustomScan is only called once while the rest of the functions are called for each scan separately. Is this expected behavior? How to work around this?
Thank you!
On Mon, Apr 17, 2023 at 3:45 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Amin <amin.fallahi@gmail.com> writes:
> To simplify: Can CustomScan scan multiple relations in the same query or it
> will always be assigned to one or zero relations?
There's barely any code in the core planner that is specific to custom
scans. Almost certainly this misbehavior is the fault of your
custom-path-creation code. Maybe you're labeling the paths with the
wrong parent relation, or forgetting to submit them to add_path,
or assigning them costs that are high enough to get them rejected?
regards, tom lane
Amin <amin.fallahi@gmail.com> writes:
> I made sure EXPLAIN returns CustomScan for all scans in the query. But
> still, ExecCustomScan is only called once while the rest of the functions
> are called for each scan separately. Is this expected behavior? How to work
> around this?
[shrug...] There's some bug in your code, which you've not shown us
(not that I'm volunteering to review it in any detail).
regards, tom lane