Обсуждение: optimal insert

Hello experts,

I have a database that contains three tables:

create table a ( id serial primary key, ... -- some more fields not relevant for my question
);
create table b ( id serial primary key, ... -- some more fields not relevant for my question
);
create table a_b ( a int not null references a, b int not null references b
);

Tables a and b have already been filled with lots of rows. Now my
application needs to insert the relationship of a to b into table a_b
and is currently doing it with inserts like the following:

insert into a_b(a,b) values(1,100);
insert into a_b(a,b) values(1,200);
insert into a_b(a,b) values(1,54);
insert into a_b(a,b) values(1,4577);

So for a batch of inserts the value of a stays the same, while for by
arbitrary values are inserted. Now I have wondered if PostreSQL offers
a smarter way to insert those values? A solution can contains usage of
some plpgsql code.

-- 
---> Dirk Jagdmann
----> http://cubic.org/~doj
-----> http://llg.cubic.org

On Sun, Oct 08, 2006 at 23:04:02 +0200, Dirk Jagdmann <jagdmann@gmail.com> wrote:
> 
> insert into a_b(a,b) values(1,100);
> insert into a_b(a,b) values(1,200);
> insert into a_b(a,b) values(1,54);
> insert into a_b(a,b) values(1,4577);
> 
> So for a batch of inserts the value of a stays the same, while for by
> arbitrary values are inserted. Now I have wondered if PostreSQL offers
> a smarter way to insert those values? A solution can contains usage of
> some plpgsql code.

I don't think you are going to be able to save anything by 'a' being fixed
unless there is a way to compute 'b', so that you can use a select statement.

Just do the normal stuff you would when speeding up inserts. The biggest
probably being wrapping them all up in one transaction.

And don't forget that \COPY and especially COPY are usually much faster
(and, IMHO, easier to compose/maintain) than gobs of INSERTs.


> -----Original Message-----
> From: pgsql-sql-owner@postgresql.org
> [mailto:pgsql-sql-owner@postgresql.org] On Behalf Of Aaron Bono
> Sent: Tuesday, October 10, 2006 1:46 PM
> To: Dirk Jagdmann
> Cc: pgsql-sql@postgresql.org
> Subject: Re: [SQL] optimal insert
>
> On 10/8/06, Dirk Jagdmann <jagdmann@gmail.com> wrote:
>
>     Hello experts,
>
>     I have a database that contains three tables:
>
>     create table a (
>     id serial primary key,
>     ... -- some more fields not relevant for my question
>     );
>     create table b (
>     id serial primary key,
>     ... -- some more fields not relevant for my question
>     );
>     create table a_b (
>     a int not null references a,
>     b int not null references b
>     );
>
>     Tables a and b have already been filled with lots of
> rows. Now my
>     application needs to insert the relationship of a to b
> into table a_b
>     and is currently doing it with inserts like the following:
>
>     insert into a_b(a,b) values(1,100);
>     insert into a_b(a,b) values(1,200);
>     insert into a_b(a,b) values(1,54);
>     insert into a_b(a,b) values(1,4577);
>
>     So for a batch of inserts the value of a stays the
> same, while for by
>     arbitrary values are inserted. Now I have wondered if
> PostreSQL offers
>     a smarter way to insert those values? A solution can
> contains usage of
>     some plpgsql code.
>
>
> It depends on your logic.  If you can write a query that
> selects out the a and b records, there is a smart way:
>
> insert into a_b(a, b)
> select 1, b.id from b where b.id in (100, 200, 54, 4577);
>
> This is not really smart because you already have the id
> values - the select may diminish your performance rather than
> help.  But if the select is "smarter" then you don't even
> need to know what the b.id <http://b.id>  values are:
>
> insert into a_b(a, b)
> select 1, b.id from b where b.somecolumn = 'somevalue';
>
> Hope this helps.
>
>
>
> ==================================================================
>        Aaron Bono
>        Aranya Software Technologies, Inc.
>        http://www.aranya.com
>        http://codeelixir.com
>
> ==================================================================
>
>

Hello Aaron,

thank you for your suggestion. I will have to think if something
similar would be of any benefit for my data.

-- 
---> Dirk Jagdmann
----> http://cubic.org/~doj
-----> http://llg.cubic.org

Hello George,

> And don't forget that \COPY and especially COPY are usually much faster
> (and, IMHO, easier to compose/maintain) than gobs of INSERTs.

I did not forget, but my application uses embedded SQL (with the epcg
preprocessor) and I don't think it can handle COPYs :(

-- 
---> Dirk Jagdmann
----> http://cubic.org/~doj
-----> http://llg.cubic.org

Hi, Dirk,

Dirk Jagdmann wrote:

> So for a batch of inserts the value of a stays the same, while for by
> arbitrary values are inserted. Now I have wondered if PostreSQL offers
> a smarter way to insert those values? A solution can contains usage of
> some plpgsql code.

[local]:postgres=# insert into a_b (a,b) SELECT 1,
(ARRAY[100,200,54,4577])[i] FROM generate_series(1,4) AS i;
INSERT 0 4

[local]:postgres=# select * from a_b;a |  b
---+------1 |  1001 |  2001 |   541 | 4577

But I tend to agree with others that packing a whole bunch of INSERTs
into a single transaction, and using COPY instead will give you better
benefits.

HTH,
Markus
-- 
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

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