On Fri, 30 Oct 2026 19:52:00 +0800, Yao Feng <[email protected]> wrote: > Hi Greg,
Hey Yao, I wanted to follow-up again on a suggestion you made. > Thanks for working on this, took a look at the v4 patch. > > I think this is very worthwhile work. FWIW, a recent SIGMOD paper on > B-tree compression (https://dl.acm.org/doi/10.1145/3654972) noted that > while front-coding saves a lot of space for strings, decompression CPU cost > tends to be the main bottleneck for queries on modern hardware. Looking at > the v4 benchmarks, they mostly cover index size and write throughput -- did > you have a chance to measure read-side performance (e.g. index scans, point > lookups)? > > A few quick questions from having played with something similar a while > back: > > 1. Skipping decompression when index tuples aren't returned: > Right now _bt_readpage() eagerly calls _bt_getitem() to decode every leaf > tuple. That's necessary when the scan actually needs to return the index > tuple (like IndexOnlyScan), but for operations that don't return tuples and > only care about the heap TID -- such as a plain IndexScan, BitmapIndexScan, > or COUNT(*) -- could we skip decompression? > > 2. Multi-column indexes: > The patch requires column 0 to be a varlena. I'm curious if you considered > supporting subsequent columns (e.g. for something like `(tenant_id int, url > text)`), or if keeping it strictly to the first column was just a natural > place to draw the line on complexity for now? This is a good idea and if I dismissed it earlier I apologize. But it will be tricky to do. Each column used to form a key may need a different key prefix, or maybe more than one can share a key prefix on a page. I'll have to think about this and do some research to find out how this has been approached by other databases in the past. > 3. Prefix boundaries within a page: > With one prefix per page, if a page spans two different domains (say, a > bunch of `http://example.com/1` <http://example.com/1>, `.../2` followed by > `http://sample.org/1` <http://sample.org/1>, `.../2`), the second group > only shares "http://";. Just curious if you've considered this kind of case? I have not, in that case you'd ideally like multiple prefixes for the same column. Then you have a different set of issues to manage, which prefix belongs to a tuple? I'll have to dig on this one too. > Curious to hear your thoughts. > > -- > Yao Feng thanks for the ideas, -greg
