On Sat, Feb 11, 2017 at 6:35 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: >>>> Why can't we rely on _bt_walk_left? >>> The reason is mentioned in comments, but let me try to explain with >>> some example. When you reach that point of code, it means that either >>> the current page (assume page number is 10) doesn't contain any >>> matching items or it is a half-dead page, both of which indicates that >>> we have to move to the previous page. Now, before checking if the >>> current page contains matching items, we signal parallel machinery >>> (via _bt_parallel_release) to allow workers to read the previous page >>> (assume previous page number is 9). So it is quite possible that >>> after deciding that current page (page number 10) doesn't contain any >>> matching tuples if we directly move to the previous page (in this case >>> it will be 9) by using _bt_walk_left, some other worker would have >>> read page 9. In short, if we directly use _bt_walk_left(), then we >>> are prone to returning some of the values twice as multiple workers >>> can read the same page. >> But ... the entire point of the seize-and-release stuff is to avoid >> this problem. You're suppose to seize the scan, read the current >> page, walk left, store the page you find in the scan, and then release >> the scan. > Exactly and that is what is done in the patch. Basically, if we found > that the current page is half-dead or it doesn't contain any matching > items, then release the current buffer, seize the scan, read the > current page, walk left and so on. I am slightly confused here > because it seems both of us agree on what is the right thing to do and > according to me that is how it is implemented. Are you just ensuring > about whether I have implemented as discussed or do you see a problem > with the way it is implemented?
Well, before, I thought you said that relying entirely on _bt_walk_left couldn't work because then two people might end up running it at the same time, and that would cause problems. But if you can only run _bt_walk_left while you've got the scan seized, then that can't happen. Evidently I'm missing something here. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
