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Maintain a simple array of metadata about pages that were deleted during nbtree VACUUM's current btvacuumscan() call. Use this metadata at the end of btvacuumscan() to attempt to place newly deleted pages in the FSM without further delay. It might not yet be safe to place any of the pages in the FSM by then (they may not be deemed recyclable), but we have little to lose and plenty to gain by trying. In practice there is a very good chance that this will work out when vacuuming larger indexes, where scanning the index naturally takes quite a while. This commit doesn't change the page recycling invariants; it merely improves the efficiency of page recycling within the confines of the existing design. Recycle safety is a part of nbtree's implementation of what Lanin & Shasha call "the drain technique". The design happens to use transaction IDs (they're stored in deleted pages), but that in itself doesn't align the cutoff for recycle safety to any of the XID-based cutoffs used by VACUUM (e.g., OldestXmin). All that matters is whether or not _other_ backends might be able to observe various inconsistencies in the tree structure (that they cannot just detect and recover from by moving right). Recycle safety is purely a question of maintaining the consistency (or the apparent consistency) of a physical data structure. Note that running a simple serial test case involving a large range DELETE followed by a VACUUM VERBOSE will probably show that any newly deleted nbtree pages are not yet reusable/recyclable. This is expected in the absence of even one concurrent XID assignment. It is an old implementation restriction. In practice it's unlikely to be the thing that makes recycling remain unsafe, at least with larger indexes, where recycling newly deleted pages during the same VACUUM actually matters. An important high-level goal of this commit (as well as related recent commits |
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