Re: Fixing Grittner's planner issues
От | Robert Haas |
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Тема | Re: Fixing Grittner's planner issues |
Дата | |
Msg-id | 603c8f070902191046g4f00203fi37d7abafc9fe2390@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: Fixing Grittner's planner issues (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: Fixing Grittner's planner issues
(Tom Lane <tgl@sss.pgh.pa.us>)
|
Список | pgsql-hackers |
On Thu, Feb 19, 2009 at 1:38 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > [ after re-reading the code a bit ] > > Robert Haas <robertmhaas@gmail.com> writes: >> Cool. On the topic of documentation, I find the following comment in >> joinrels.c rather impenetrable: > >> /* >> * Do these steps only if we actually have a >> regular semijoin, >> * as opposed to a case where we should >> unique-ify the RHS. >> */ > > The point here is that a semijoin ordinarily requires forming the whole > LHS relation (ie, min_lefthand) before applying the special join rule. > However, if you unique-ify the RHS then it's a regular inner join and > you don't have to obey that restriction, ie, you can join to just part > of min_lefthand. Now ordinarily that's not an amazingly good idea but > there are important special cases where it is. IIRC the case that > motivated this was > > SELECT FROM a, b WHERE (a.x, b.y) IN (SELECT c1, c2 FROM c) > > If you do this as a semijoin then you are forced to form the cartesian > product of a*b before you can semijoin to c. If you uniqueify c > then you can join it to a first and then b using regular joins (possibly > indexscans on a.x and then b.y), or b and then a. > > So join_is_legal allows such a join order, and the code in make_join_rel > has to be careful not to claim that "a semijoin c" is a valid way of > forming that join. Gotcha. > I'll change the comment. Does this help? > > /* > * We might have a normal semijoin, or a case where we don't have > * enough rels to do the semijoin but can unique-ify the RHS and > * then do an innerjoin. In the latter case we can't apply > * JOIN_SEMI joining. > */ It's an improvement, but your example above is so helpful in understanding what is going on here that it might be worth explicitly mentioning it in the comment, maybe something like this: /** In a case like the following, we don't have enough rels to plan this as a semijoin,* but we don't give up completely, because it might be possible to unique-ify the* RHS and perform part of the join at this level.** SELECT FROM a, b WHERE (a.x, b.y) IN (SELECT c1, c2 FROMc)*/ ...Robert
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