> 14 июля 2018 г., в 0:28, Heikki Linnakangas <hlinn...@iki.fi> написал(а): > > On 13/07/18 21:28, Andrey Borodin wrote: >>> 13 июля 2018 г., в 18:25, Heikki Linnakangas <hlinn...@iki.fi> >>> написал(а): >>> Looking at the second patch, to scan the GiST index in physical >>> order, that seems totally unsafe, if there are any concurrent page >>> splits. In the logical scan, pushStackIfSplited() deals with that, >>> by comparing the page's NSN with the parent's LSN. But I don't see >>> anything like that in the physical scan code. >> Leaf page can be pointed by internal page and rightlink >> simultaneously. The purpose of NSN is to visit this page exactly once >> by following only on of two links in a scan. This is achieved >> naturally if we read everything from the beginning to the end. (That >> is how I understand, I can be wrong) > > The scenario where this fails goes like this: > > 1. Vacuum scans physical pages 1-10 > 2. A concurrent insertion splits page 15. The new left half stays on page 15, > but the new right half goes to page 5 > 3. Vacuum scans pages 11-20 > > Now, if there were any dead tuples on the right half of the split, moved to > page 5, the vacuum would miss them. > > The way this is handled in B-tree is that when a page is split, the page is > stamped with current "vacuum cycle id". When the vacuum scan encounters a > page with the current cycle id, whose right-link points to a lower-numbered > page, it immediately follows the right link, and re-scans it. I.e. in the > above example, if it was a B-tree, in step 3 when vacuum scans page 15, it > would see that it was concurrently split. It would immediately vacuum page 5 > again, before continuing the scan in physical order. > > We'll need to do something similar in GiST.
OK, I will do this. This is tradeoff between complex concurrency feature and possibility of few dead tuples left after VACUUM. I want to understand: is it something dangerous in this dead tuples? There is one more serious race condition: result of first scan is just a hint where to look for downlinks to empty pages. If internal page splits between scan and cleanup, offsets of downlinks will be changed, cleanup will lock pages, see non-empty pages and will not delete them (though there are not dead tuples, just not deleted empty leafs). Best regards, Andrey Borodin.
