We have to confirm the theory first: a 'perf top' sampling during two runs shall give enough information. Regards, Qingqing On Tue, Mar 31, 2015 at 8:58 AM, Kevin Viraud <kevin.viraud@xxxxxxxxxxxxxxxxxx> wrote: > Touche ! Thanks a lot. > > Looking more at the data yes it goes very often to ELSE Clause. And > therefore reaching the MAX_CACHED_RES. > > In there anyway to increase that value ? > > Basically, I have several tables containing millions of rows and let say 5 > columns. Those five columns, depending of their combination give me a 6th > value. > We have complex patterns to match and using simple LIKE / EQUAL and so on > wouldn't be enough. This can be applied to N number of table so we > refactored this process into a function that we can use in the SELECT > statement, by giving only the 5 values each time. > > I wouldn't mind using a table and mapping it through a join if it were for > my own use. > But the final query has to be readable and usable for almost-non-initiated > SQL user... So using a function with encapsulated case when seemed to be a > good idea and so far worked nicely. > > But we might consider changing it if we have no other choice... > > Regards, > > Kevin > > -----Original Message----- > From: Tom Lane [mailto:tgl@xxxxxxxxxxxxx] > Sent: Dienstag, 31. März 2015 15:59 > To: Kevin Viraud > Cc: pgsql-performance@xxxxxxxxxxxxxx > Subject: Re: Weird CASE WHEN behaviour causing query to be > suddenly very slow > > "Kevin Viraud" <kevin.viraud@xxxxxxxxxxxxxxxxxx> writes: >> I have an issue with a rather large CASE WHEN and I cannot figure out >> why it is so slow... > > Do all the arms of the CASE usually fail, leaving you at the ELSE? > > I suspect what's happening is that you're running into the MAX_CACHED_RES > limit in src/backend/utils/adt/regexp.c, so that instead of just compiling > each regexp once and then re-using 'em, the regexps are constantly falling > out of cache and then having to be recompiled. They'd have to be used in a > nearly perfect round robin in order for the behavior to have such a big > cliff as you describe, though. In this CASE structure, that suggests that > you're nearly always testing every regexp because they're all failing. > > I have to think there's probably a better way to do whatever you're trying > to do, but there's not enough info here about your underlying goal to > suggest a better approach. At the very least, if you need a many-armed > CASE, it behooves you to make sure the common cases appear early. > > regards, tom lane > > > > -- > Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
