On Tue, Dec 20, 2016 at 7:44 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Tue, Dec 20, 2016 at 9:01 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: >> On Tue, Dec 20, 2016 at 7:11 PM, Robert Haas <robertmh...@gmail.com> wrote: >>> On Tue, Dec 20, 2016 at 4:51 AM, Amit Kapila <amit.kapil...@gmail.com> >>> wrote: >>>> We have mainly four actions for squeeze operation, add tuples to the >>>> write page, empty overflow page, unlinks overflow page, make it free >>>> by setting the corresponding bit in overflow page. Now, if we don't >>>> log the changes to write page and freeing of overflow page as one >>>> operation, then won't query on standby can either see duplicate tuples >>>> or miss the tuples which are freed in overflow page. >>> >>> No, I think you could have two operations: >>> >>> 1. Move tuples from the "read" page to the "write" page. >>> >>> 2. Unlink the overflow page from the chain and mark it free. >>> >>> If we fail after step 1, the bucket chain might end with an empty >>> overflow page, but that's OK. >> >> If there is an empty page in bucket chain, access to that page will >> give an error (In WAL patch we are initializing the page instead of >> making it completely empty, so we might not see an error in such a >> case). > > It wouldn't be a new, uninitialized page. It would be empty of > tuples, not all-zeroes. >
AFAIU we initialize page as all-zeros, but I think you are envisioning that we need to change it to a new uninitialized page. >> What advantage do you see by splitting the operation? > > It's simpler. The code here is very complicated and trying to merge > too many things into a single operation may make it even more > complicated, increasing the risk of bugs and making the code hard to > maintain without necessarily buying much performance. > Sure, if you find that way better, then we can change it, but the current patch treats it as a single operation. If after looking the patch you find it is better to change it into two operations, I will do so. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
