On 27 November 2017 at 05:53, Tom Lane <[email protected]> wrote: > Simon Riggs <[email protected]> writes: >> On 27 November 2017 at 04:46, Tom Lane <[email protected]> wrote: >>> Well, I'm concerned about the possibility of a lot of palloc thrashing >>> if the first bunch of records it reads happen to have steadily increasing >>> sizes. However, rather than doubling, it might be sufficient to set a >>> robust minimum on the first allocation, ie use something along the lines >>> of Max(1024, MAXALIGN(state->main_data_len)). > >> Agreed. > >> I was just researching what that number should be... and I was >> thinking that we should use the maximum normal tuple size, which I >> think is > >> TOAST_TUPLE_THRESHOLD + >> SizeOfXLogRecord + >> SizeOfXLogRecordDataHeaderLong > > Well, let's not overthink this, because anything under 8K is going to > be rounded up to the next power of 2 anyway by aset.c. Based on this > point I'd say that BLCKSZ/2 or BLCKSZ/4 would be reasonable candidates > for the minimum.
BLCKSZ/2 seems best then. I guess that means palloc doesn't work well with BLCKSZ > 8192 -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
