Re: Question on overall design

On Tue, Dec 12, 2023 at 2:11 AM Ron Johnson <ronljohnsonjr(at)gmail(dot)com> wrote:

> On Mon, Dec 11, 2023 at 4:41 AM Dominique Devienne <ddevienne(at)gmail(dot)com>
> wrote:
>
>> On Sun, Dec 10, 2023 at 5:56 PM Ron Johnson <ronljohnsonjr(at)gmail(dot)com>
>> wrote:
>>
>>> * We departitioned because SELECT statements were *slow*. All
>>> partitions were scanned, even when the partition key was specified in the
>>> WHERE clause.
>>>
>>
>> Surely that's no the case on newer PostgreSQL, is it? Otherwise what's
>> the point of partitioning?
>> Also, I remember reading something about recent improvements with a large
>> number of partitions, no?
>>
>> As someone who's interested on partitioning, I'd appreciate details.
>> Thanks, --DD
>>
>
> This was on 12.5. v13 was just released, and we weren't confident about
> running a mission-critical system on a .1 version.
>

Something's wrong if all partitions are scanned even when the partition
clause is explicit in the where clause.

There are however some things which can cause problems here, such as type
casts of the partition key, or when the partition key is being brought in
from a join.

>
> All "transaction" tables were partitioned by month on partion_date, while
> the PK was table_name_id, partition_date.
>
> Queries were _slow_, even when the application knew the partion_date range
> (since queries might span months). PG just wouldn't prune.
>

Was there a datatype issue here? Like having a partition key of type
timestamp, but the query casting from date?

>
> When I departitioned the tables, performance became acceptable.
>
>
>

--
Best Wishes,
Chris Travers

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