Other indexes do bloat, but the percentage bloat is a lot less, presumably because this is a partial index where the partial column has a high degree of changes ie maybe 100 genuinely ‘live’ rows in a table of 300 million where every row has gone through a state where it would have been in the index. In some of our partitions we might have 2000 old rows that do hang around for a long time and another 100 or so ‘real’ partial index entries so 2200 in total but the number of rows would be 300 million so it is a lot less than 1%.
> On 16 Jul 2021, at 16:43, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Tom Dearman <tom.dear...@gmail.com> writes: >> We have change autovacuum so that it runs more frequently >> autovacuum_vacuum_scale_factor=0.035, the reason we have a partial index on >> the status is that in a table of 300 million entries, only about 100 or so >> would have status=‘IN_PROGRESS’ so we think this should be a nice small >> index and many of our queries want to look up with a where clause >> status=‘IN_PROGRESS’. In theory it works well, but we get a lot of index >> bloat as there is a lot of churn on the status value, ie each row starts as >> IN_PROGRESS and then goes to one of 4 possible completed statuses. > > Is it really the case that only this index is bloating? In principle, an > update on a row of the table should result in new entries in every index > of the table. A partial index, due to the filter applied to possibly not > store any index entry, should in theory have less bloat than other > indexes. > > If that's not what you're seeing, there must be something about the data > being stored in that index (not the partial-index filter condition) that > results in a lot of low-occupancy index pages over time. You didn't say > anything about what the data payload is. But we've seen bloat problems in > indexes where, say, every tenth or hundredth value in the index ordering > would persist for a long time while the ones in between get deleted > quickly. That leads to low-density indexes that VACUUM can't do anything > about. > > regards, tom lane
