> -----Original Message----- > [mailto:pgsql-general-owner@xxxxxxxxxxxxxx] On Behalf Of Tom Lane > > Tzahi Fadida <tzahi_ml@xxxxxxxxxxxxx> writes: > > I am writing an algorithm in a dynamic c library and using > heap_fetch. > > I want to pin strategic buffers for long times like an Outer joins > > might do for the inner table. > > > Do i to also need to lock the table somehow? > > You didn't say enough about the context. It is *always* > necessary to have some type of lock on a table you are > accessing --- otherwise some other backend could drop the > table out from under you. But in many scenarios this is > taken care of at a pretty high level, like executor startup. > What is your "algorithm" and when is it applied? How does it > get the table to be accessed? Do I maintain a lock on a table with heap_open? I was also meaning to ask if I need to also lock the buffers I am pinning? I am writing a full-disjunctions algorithm that is really like a natural outer join for 2 relations and usually has no equality in more than 2 relations. let noj = natural outer join. E.g. FD(A,B) will give the same as A noj B. but FD(A,B,C) might not be as A noj B noj C and the FD alg is here to correct problems of duplicate answers in A noj B noj C. So, as with a full natural outer join I can't have the tables be dropped under me in the middle, but all the operation is read only. Example of usage: SELECT * FROM FD(A,B,C) I am really building the prototype now, no one has ever implemented FD so I don't want to put it just yet in the executor which is why I am writing a dynamic c library function. > > > I am only reading the tuple but maybe other transactions > will want to > > write to it and when I am looping over the table like in an > outer join > > does I can get different values and I want to avoid that. > > or that can't happen when pinning? > > Pinning has nothing to do with that --- maintaining a > consistent snapshot setting does. How do you maintain a consistent snapshot settings with heap_fetch? with heap_open and heap_beginscan I just save it to a context. do I use them here before heap_fetch the same way I would do for heap_getnext? > > > Another question, when a column attribute is toasted, > > do I need to do another heap_fetch aside from the main > > table to fetch the data from the adjoined toasted table? > > How do I also pin the toasted table buffer pages? > > No, you do a DETOAST_DATUM call. > > > And last general question, > > what is the best way to compare two datums? > > datumIsEqual doc says its not enough because of > > different representations, and if its toasted it won't > > work (maybe if its detoasted first). > > You really should locate the datatype's equality function and > call that. datumIsEqual is a kluge that is only safe to use > in the very limited context of equalfuncs.c (basically, it's > OK to say that two Const nodes are different if they are > bitwise different, even if the represented values are > logically equal). > > Look at array_eq() for an example of current best practice for this. > > regards, tom lane > > ---------------------------(end of > broadcast)--------------------------- > TIP 3: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to majordomo@xxxxxxxxxxxxxx so > that your > message can get through to the mailing list cleanly > > ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@xxxxxxxxxxxxxx
