On 8 January 2018 at 13:35, Amit Kapila <amit.kapil...@gmail.com> wrote: > On Mon, Jan 8, 2018 at 11:26 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> Amit Khandekar <amitdkhan...@gmail.com> writes: >>> The fact that b_star gets moved from 5th position to the first >>> position in the scans, indicates that it's cost shoots up from 1.04 to >>> a value greater than 1.16. It does not look like a case where two >>> costs are almost same due to which their positions swap sometimes. I >>> am trying to figure out what else can it be ... >> > > That occurred to me as well, but still, the change in plan can happen > due to the similar costs.
Agreed. But I think we should first fix the issue due to which the test might be failing in this case. BTW, for your patch, I am thinking we can have a separate factor other than STD_FUZZ_FACTOR ? This way, we can make it much smaller than 1.01 also. And anyways, STD_FUZZ_FACTOR is used only for comparing paths on the same relation, whereas in our case, our comparision goal is different. > Another possibility as indicated in the > previous email is that if somehow the stats of table (reltuples, > relpages) is not appropriate, say due to some reason analyze doesn't > happen on the table. Yes, I am also thinking on the same lines. E.g., if the relpages is 0 (due to no analyze run yet), tuple density calculation follows a different logic, due to which reltuples can be quite bigger. I suspect this also might be the reason. So yes, I think it's worth having ANALYZE on *_star. > For example, if you just manually update the > value of reltuples for b_star in pg_class to 20 or so, you will see > the plan as indicated in the failure. If that is true, then probably > doing Analyze before Parallel Append should do the trick. Or better still, we can have Analyze in create_misc.sql and create_table.sql where the table is populated. > >> The gut feeling I had upon seeing the failure was that the plan shape >> depends on the order in which rows happen to be read from the system >> catalogs by a heapscan. I've not tried to run that idea to ground yet. >> > > I don't see how something like that can happen because we internally > sort the subpaths for parallel append. True. Or may I didn't understand. Tom, are you referring to reading pg_class.reltuples or similar, when say "read from the system catalogs" ? > > > -- > With Regards, > Amit Kapila. > EnterpriseDB: http://www.enterprisedb.com -- Thanks, -Amit Khandekar EnterpriseDB Corporation The Postgres Database Company
