| Lists: | pgsql-hackers |
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| From: | Parul Lakkad <parul(dot)lakkad(at)gmail(dot)com> |
|---|---|
| To: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Disk usage for intermediate results in join |
| Date: | 2014-03-11 11:24:57 |
| Message-ID: | CACmw6hSNjttk2uhP7KV-OWQ3Lv2KdeB8TdOva7J1EgMtLxzzug@mail.gmail.com |
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| Lists: | pgsql-hackers |
Hi,
I am trying to figure out when disk is used to store intermediate results
while performing joins in postgres.
According my findings using 'explain analyse ' only merge sort uses disk.
Can anyone please throw some more light on this?
Thanks,
Parul
| From: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
|---|---|
| To: | Parul Lakkad <parul(dot)lakkad(at)gmail(dot)com> |
| Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Disk usage for intermediate results in join |
| Date: | 2014-03-11 12:22:45 |
| Message-ID: | 531F0015.4000200@vmware.com |
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On 03/11/2014 01:24 PM, Parul Lakkad wrote:
> Hi,
>
> I am trying to figure out when disk is used to store intermediate results
> while performing joins in postgres.
>
> According my findings using 'explain analyse ' only merge sort uses disk.
> Can anyone please throw some more light on this?
Hash joins will also spill to disk if the hash-side of the join is large
enough. The planner usually tries to avoid it, but sometimes it happens.
- Heikki
| From: | Marti Raudsepp <marti(at)juffo(dot)org> |
|---|---|
| To: | Parul Lakkad <parul(dot)lakkad(at)gmail(dot)com> |
| Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Disk usage for intermediate results in join |
| Date: | 2014-03-14 15:54:26 |
| Message-ID: | CABRT9RBSNMq+eWEJsp70pN95+O3B2ftypzpcfZyGevWA8bY03Q@mail.gmail.com |
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On Tue, Mar 11, 2014 at 1:24 PM, Parul Lakkad <parul(dot)lakkad(at)gmail(dot)com> wrote:
> I am trying to figure out when disk is used to store intermediate results
> while performing joins in postgres.
Joins can also cause a Nested Loop+Materialize plan, which spills to
disk if the materialize result set is too large for work_mem.
Regards,
Marti