Re: Declarative partitioning, UUIDs, index issues.

Hi Oliver,

I know I am not addressing your immediate concern about why it is not
using the index, but I have another question first:

You are using a query that is not doing any partition exclusion since
you are not providing the date on the where clause, which is the
partitioning key, right?  If so, I would expect to see all the
partitions in the explain output, but I only see 4 thru 8.

Regards,
Michael Vitale

Wells Oliver wrote on 1/13/2020 12:21 PM:
> Hi all. I have a table with 7 columns: uuid, date, smallint, smallint,
> real, real, real.
>
> The PK is the uuid, date, smallint, smallint.
>
> It has 60 partitions on date, partitioned by month, back to 2015. Each
> partition is roughly 40GB, about 350-450m rows.
>
> Doing a join to this table using the uuid is not using an index, so
> it's never completing. So where this table is b: SELECT * FROM a JOIN
> b ON A.uuid = B.uuid - I thought it might use the PK as the uuid is
> the first column, but we end up with a ton of sequence scans:
>
>  Hash Join  (cost=113773.34..326420199.08 rows=1187847 width=46)
>    Hash Cond: (a.uuid = myt.uuid)
>    ->  Append  (cost=0.00..282743385.62 rows=11613643108 width=42)
>          ->  Seq Scan on myt_2015_05 myt_4  (cost=0.00..7647300.32
> rows=395295232 width=42)
>          ->  Seq Scan on myt_2015_06 myt_5  (cost=0.00..7233480.44
> rows=373904544 width=42)
>          ->  Seq Scan on myt_2015_07 myt_6  (cost=0.00..6679997.60
> rows=345294560 width=42)
>          ->  Seq Scan on myt_2015_08 myt_7  (cost=0.00..7426294.64
> rows=383871264 width=42)
>          ->  Seq Scan on myt_2015_09 myt_8  (cost=0.00..7454691.04
> rows=385339104 width=42)
>
> Etc, all the way up to present. I then tried adding a btree index on
> the uuid itself, same result.
>
> Selecting directly on the table where the uuid = some value will use
> the index, but joining to the table will NOT.
>
> I ran analyze on the primary table, still a sequence scan.
>
> This table is useless without an index on the uuid, what might be the
> issue? Might my partitions be too large? Something else?
>
> --
> Wells Oliver
> wells(dot)oliver(at)gmail(dot)com <mailto:wellsoliver(at)gmail(dot)com>