Effects of GUC settings on automatic replans - Mailing list pgsql-hackers
From | Tom Lane |
---|---|
Subject | Effects of GUC settings on automatic replans |
Date | |
Msg-id | 15168.1174410673@sss.pgh.pa.us Whole thread Raw |
Responses |
Re: Effects of GUC settings on automatic replans
(Josh Berkus <josh@agliodbs.com>)
Re: Effects of GUC settings on automatic replans (Jan Wieck <JanWieck@Yahoo.com>) Re: Effects of GUC settings on automatic replans (Jim Nasby <decibel@decibel.org>) |
List | pgsql-hackers |
Now that there's a mechanism in the backend that will automatically replan queries whenever anything changes about the referenced tables, we have to worry about whether an automatic replan might cause surprising changes in the behavior of a query. I looked through the available GUC settings to see what would affect a replan, and came up with just four that would potentially affect the semantics of the query: search_pathadd_missing_fromtransform_null_equalssql_inheritance As I've already mentioned, I think we must address search_path by saving the path at time of first plan and using that same path during any replan. However, I'm not excited about adding mechanism to similarly save and restore the others. They're all for legacy-app compatibility and so seem unlikely to be changed on-the-fly within a session. Also, add_missing_from and transform_null_equals aren't going to affect sanely written queries in the first place. sql_inheritance is a little bit bigger deal, but I wonder whether we shouldn't just remove that variable altogether --- it's been default ON since 7.1 and I've not heard anyone complain about that in a long time. There are a boatload of other GUCs that could potentially result in changes of planner choices: enable_bitmapscanenable_hashaggenable_hashjoinenable_indexscanenable_mergejoinenable_nestloopenable_seqscanenable_sortenable_tidscanconstraint_exclusionfrom_collapse_limitjoin_collapse_limitgeqogeqo_effortgeqo_generationsgeqo_pool_sizegeqo_selection_biasgeqo_thresholdseq_page_costrandom_page_costcpu_tuple_costcpu_index_tuple_costcpu_operator_costeffective_cache_sizework_mem I'm inclined not to worry about these, since changing them can't affect the semantics of the query, at worst its performance. One other question is exactly what "saving and restoring" search_path should mean. We could do it textually and thus need to re-interpret the string on each replan, or we could save the actual list of schema OIDs. The main disadvantage of the textual way is that without some special hack, it's possible that a replan would see the temp-table schema as being frontmost when it had not been active at all originally; that seems bad. OTOH if we save the OID list then it would not work to drop a schema and rename another into its place, which is a bit inconsistent with the fact that that does work for an individual table. Comments? regards, tom lane
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