Message-ID: <f2212e575cef69c6f3c87fbb57f1cf44bff63ee7.camel@cybertec.at>
Subject: Re: Oracle to Postgres - Transform Hash Partition
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: David Barbour <dbarbour@istation.com>, pgsql-general@lists.postgresql.org
Date: Fri, 07 Jun 2024 10:31:16 +0200
In-Reply-To: <CAEMHB2R1VA09f22WbBOJz5YE6WLC7aK7HYwo7UXSWdyqN5bmpw@mail.gmail.com>
References: 
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On Thu, 2024-06-06 at 11:28 -0500, David Barbour wrote:
> Been an Oracle DBA for quite a while and we're moving from Oracle to Post=
gres.
>=20
> I have a table that I need to transform into a hash partitioned table.
> I've gone through all the steps to create a hash partitioned table, with =
8 partitions such as=C2=A0
>=20
> create table idev.assessment_result_2023_dab_part (like idev.assessment_r=
esult_2023_dab)partition by hash (district_oid);
>=20
> CREATE TABLE idev.assessment_result_2023_dab_part_assessment_result_2023_=
p1 PARTITION OF idev.assessment_result_2023_dab_part
> =C2=A0 =C2=A0 FOR VALUES WITH (modulus 64, remainder 0)
>=20
> ....etc through partition 2023_p8 FOR VALUES WITH (modulus 64, remainder =
7)

That won't do.  If you use a modulus of 64, you need 64 partitions, one for
each possible division remainder.

If you want 8 partitions, you have to use modulus 8.

> Now I need to 'attach' the original table.=C2=A0 The problem I'm running =
into is
> there are no good examples of how to define the values.=C2=A0=C2=A0
>=20
> I've tried several iterations of various 'for values', 'values', 'for val=
ues with',
> etc. but they all error out.
>=20
> Here's an example:
> =C2=A0alter table idev.assessment_result_2023_dab_part=20
> =C2=A0attach partition idev.assessment_result_2023_dab for values with(mo=
dulus 8, remainder 1) to (modulus 8, remainder 7)
>=20
> ERROR: syntax error at or near "to" LINE 2: ..._2023_dab for values with(=
modulus 8, remainder 1) to (modulu...

I think you are confused about hash partitioning.

One partition is only for a single remainder.  You cannot have a partition =
for
several remainders.

Hash partitioning is mostly for splitting up a table into several parts of
roughly equal size.  You decide how many partitions you want; that will bec=
ome
the modulus.  Then you have to create that many partitions, one for each re=
mainder.

If you want to attach an existing table as a partition, that will only work=
 if
all rows in the table belong into that partition.  Otherwise, you will get =
an
error.
So you typically won't be able to attach an existing table as a hash partit=
ion.

To convert an existing table into a hash partitioned table, you have to

- create a new, empty partitioned table with *all* its partitions

- transfer the data with "INSERT INTO hash_part_tab SELECT * FROM tab"

Yours,
Laurenz Albe