Thanks for all the answers.
> add autoincrement=False to the Column
This is actually all I needed, but possibly my findings can help others or
provide improvements.
I'm not using drop_alll and create_all, but table.create() and this results
in the "relation "some_seq" already exists" error.
99% your snippet, just using create:
from sqlalchemy import Column
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import Sequence
from sqlalchemy import Table
m = MetaData()
seq = Sequence("some_seq")
t1 = Table("t1", m, Column("id", Integer, seq, primary_key=True))
t2 = Table("t2", m, Column("id", Integer, seq, primary_key=True))
t1.create(pg_engine)
t2.create(pg_engine)
> To render "IF NOT EXISTS" then just invoke the SQL:
conn.execute(text("CREATE SEQ IF NOT EXISTS ..."))
So in this example, how could I avoid creating the sequence? I'd be happy
to create it once, or not at all, but it seems that table create tries to
create the sequence automatically, no matter what.
The schema bug was actually the behaviour of the optional=True. I still
don't know what it does, but it behaves like if the whole sequence line
wouldn't be present. It just makes the column a SERIAL type and then
PostgreSQL auto-creates the sequence in the table's schema, not in public.
So with optional=True basically nothing matters, nor the sequence's name,
not the sequence's schema?
Zsolt
On Wednesday, 5 August 2020 at 17:36:00 UTC+2 Mike Bayer wrote:
>
>
> On Wed, Aug 5, 2020, at 9:25 AM, Zsolt Ero wrote:
>
> But this would create a different id for each table, wouldn't it?
>
>
>
> if you want two tables to have the same sequence then use one Sequence
> object for both. If the Sequence is present on the Column it will not
> create SERIAL. I've tried to document this right at the top here:
> https://docs.sqlalchemy.org/en/13/dialects/postgresql.html#sequences-serial-identity
>
> . Here's a POC with all the things I think you've mentioned so far:
>
> from sqlalchemy import Column
> from sqlalchemy import create_engine
> from sqlalchemy import Integer
> from sqlalchemy import MetaData
> from sqlalchemy import Sequence
> from sqlalchemy import Table
>
>
> m = MetaData(schema="test_schema")
>
> seq = Sequence("some_seq", schema="public")
> t1 = Table(
> "t1",
> m,
> Column(
> "id", Integer, seq, primary_key=True
> ),
> )
>
> t2 = Table(
> "t2",
> m,
> Column(
> "id", Integer, seq, primary_key=True
> ),
> )
>
> e = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
>
> m.drop_all(e)
> m.create_all(e)
>
> with e.connect() as conn:
> conn.execute(t1.insert())
> conn.execute(t2.insert())
>
> assert conn.scalar(seq.next_value()) == 3
>
>
>
>
>
>
> I'd
> like to use the same ids for matching rows, such that table A's
> primary key is the same as table B's primary key, so that they can
> join-ed together like when it was a single table.
>
> So far the only solution I found is to remove primary_key=True and
> issue an ALTER TABLE ... ADD PRIMARY KEY (...) command manually.
>
>
>
> On Wed, 5 Aug 2020 at 15:20, Mike Bayer <[email protected]> wrote:
> >
> >
> >
> > On Wed, Aug 5, 2020, at 8:59 AM, Zsolt Ero wrote:
> >
> > Hi,
> >
> > I've split a table into two tables, for performance reasons. I'd like to
> insert into both tables using the same sequence. I'm inserting using
> executemany_mode='values'.
> >
> > My idea is to call nextval() on the sequence before insert and fill in
> the values client side, before inserting.
> >
> > select nextval('mysql') FROM generate_series(1,...)
> >
> > Everything looks good, except for the default behaviour of SQLAlchemy to
> turn an integer + pk column into a SERIAL.
> >
> >
> > add autoincrement=False to the Column
> >
> >
> >
> > As an alternative I'm also looking at using Sequence('myseq') from
> https://docs.sqlalchemy.org/en/13/dialects/postgresql.html#sequences-serial-identity,
>
> but this is broken for issuing "CREATE SEQUENCE myseq" before the table
> creation, which is missing the IF NOT EXISTS part.
> >
> >
> > there's a "checkfirst" flag on Sequence.create() or Table.create() that
> will check for the object ahead of time, using a separate query. To render
> "IF NOT EXISTS" then just invoke the SQL: conn.execute(text("CREATE SEQ IF
> NOT EXISTS ..."))
> >
> >
> >
> >
> > How can I either:
> > - turn off the automatic behaviour of making a pg + int = serial?
> > - add a IF NOT EXISTS to the Sequence()?
> >
> > Or any alternative ideas?
> >
> >
> > --
> > SQLAlchemy -
> > The Python SQL Toolkit and Object Relational Mapper
> >
> > http://www.sqlalchemy.org/
> >
> > To post example code, please provide an MCVE: Minimal, Complete, and
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>
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>
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