I added code to inline_function to stop inlining if a parameter expression to be substituted multiple times has cost greater than 10*cpu_operator_cost (which roughly means that it contains more than 10 operators or functions). This seems to cut off the problem nicely, at least for this example. The factor of 10 is a bit of a magic number but it seems reasonable.
That did it (on fresh copy of cvs):
regression=# explain analyze select concat_ws('~','01','02','03','04','05','06','07','08','09','10','11','12','13','14','15','16','17','18','19','20','21','22','23','24','25','26','27','28','29','30','31');
QUERY PLAN
----------------------------------------------------------------------------------
Result (cost=0.00..0.11 rows=1 width=0) (actual time=2.37..2.37 rows=1 loops=1)
Total runtime: 2.66 msec
(2 rows)
Thanks Tom!
Joe
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