Re: how to know whether query data from memory after pg_prewarm

@Thomas, this tool looks very interesting!

@Jimmy:

Back to the question,you might approach the problem from a different perspective..

If you run a query 2 consecutive times, from the secondon, you should be at 'full speed'.

Therefore if the first run takes X seconds but the consecutive runs take only a fraction of it, then you have probably cached all the data.

It might be as simple as that. Butalsotake into account:

- What is written above is true when the size of the table fits in RAM
- No other activity is going on. If you have other activity going on on the disk, it will pollute your results
- If you read from disk, you will see read activity on the data disk, given that your are graphing it. If is cached then it does not read from disk.
- Running the query with 'explain' will tell you in detail what is going on. (Maybe you forgot to create an index on Postgres and is there on Oracle?)
- If you are doing sortsand the data does not fit on work_mem then you are making use of disk space, slowing down operations

regards,

fabio pardi

On 19/09/18 05:29, Thomas Munro wrote:
> On Wed, Sep 19, 2018 at 1:35 PM jimmy <mpokky(at)126(dot)com> wrote:
>> I use select pg_prewarm('table1','read','main') to load data of table1 into the memory.
>> when I use select count(1) from table1 group by aa to query data.
>> I find the speed of query is not fast, I wonder whether it query data from memory.
>> And it is slower than Oracle, both of Oracle and Postgresql has same table and count of data.
>> when pg_prewarm use 'read' mode, the data is put into the OS cache, how to examine the table which is pg_prewarmed into the OS cache .
>> I know pg_buffercache ,but it just examine the table in the shared buffer of Postgresql, not the table in the OS cache.
> This is a quick and dirty hack, but it might do what you want:
>
> https://github.com/macdice/pgdata_mincore
>
> Tested on FreeBSD, not sure how well it'll travel.
>