On Wed, Dec 16, 2015 at 11:41 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Robert Haas <robertmhaas@gmail.com> writes: > I like option #2. I don't really have a strong reason for that, but > it feels intuitive to me that we err on the side of using remote > estimates when in doubt.
If we believe that, why isn't the default value of use_remote_estimate true? (Maybe it should be.)
Another option that should be considered is that joins should pay attention only to the server-level setting and not to the individual tables' settings. This would surely be cheaper to implement, and it avoids any questions about whether to OR or AND the individual settings.
To implement server-level setting, we will need to pull it out of ForeignServer structure like 442 foreach(lc, fpinfo->server->options) 443 { 444 DefElem *def = (DefElem *) lfirst(lc); 445 446 if (strcmp(def->defname, "use_remote_estimate") == 0) 447 fpinfo->use_remote_estimate = defGetBoolean(def); ... 455 }
whereas use_remote_estimate setting for joining relations is readily available in PgFdwRelationInfo
58 /* Options extracted from catalogs. */ 59 bool use_remote_estimate; 60 Cost fdw_startup_cost; 61 Cost fdw_tuple_cost; 62 List *shippable_extensions; /* OIDs of whitelisted extensions */ ... 76 } ;
This use_remote_estimate is true if server level option is true or table level option is true.
ANDing or ORing use_remote_estimate from the joining relations' PgFdwRelationInfo looks cheaper than pulling it out of ForeignServer structure.
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Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
