----- Mensaje original ----- > De: "Haider Ali" <alihaider907@xxxxxxxxx> > Para: pgsql-performance@xxxxxxxxxxxxxx > Enviados: Viernes, 7 de Abril 2017 11:56:53 > Asunto: Understanding PostgreSQL query execution time > > > Hello > > > I want to understand execution time of a query in PostgreSQL then I > want to relate it to the problem i am getting. According to my > observation ( I can't explain why this happen ) whenever we query a > table first time its execution will be high (sometimes very high) as > compare to queries made on same table in a short period of time > followed by first query on that table. For example query given below > > > SELECT "global_configs".* FROM "global_configs" ORDER BY > "global_configs"."id" ASC LIMIT $1 > > > > > executed multiple times instantaneous one after another have > following execution time > > > 1st time => 147.5ms > > 2nd time => 3.0ms > > 3rd time => 3.0ms > > 4th time => 3.0ms > > 5th time => 0.8ms That is the effects of the postgres/Linux cache for shure. > > > I want to understand why there is a huge time difference between 1st > and rest of the executions. > > > Relation to other problem > > > Having experience above behaviour of PostgreSQL now I am using > PostgreSQL managed by Amazon RDS. Observation is no matter how many > times I execute same query its execution times remain same ( > although execution time of a query on RDS is comparatively high as > compare to query running on local instance of PostgreSQL that I can > understand is because of Network latency) > > > Questions > > > > > 1. Why first query on a table takes more time then queries > followed by it ? > 2. Why above behaviour doesn't reflect on Amazon RDS ? > Amazon provides you with SSD like disks, running close to memory speed. That would explain the little impact of having a ram cache. HTH Gerardo > > Haider Ali -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
