On Mon, Apr 30, 2018 at 3:13 PM, Andres Freund <and...@anarazel.de> wrote: > On 2018-04-30 14:59:31 +1200, Thomas Munro wrote: >> In EXPLAIN (BUFFERS), there are two kinds of cache misses that show up >> as "reads" when in fact they are not reads at all: >> >> 1. Relation extension, which in fact writes a zero-filled block. >> 2. The RBM_ZERO_* modes, which provoke neither read nor write. > > Just for understanding: 2) can never happen for buffers showing up in > EXPLAIN, right? > > I'm not saying you shouldn't fix the accounting...
Maybe the hash AM can reach that in _hash_getinitbuf() while adding overflow pages to bucket chains? Admittedly case 2 is obscure and rare if not unreachable and probably no one would care too much about that in practice (whereas case 1 can be seen by simply inserting stuff into a new empty table). Other patches I'm working on for later proposal do it more often (think accessing known-empty pages in a kind of preallocated extent), and it occurred to me that it's clearly a bug on principle, hence this patch. -- Thomas Munro http://www.enterprisedb.com
