Postgresql – How to Add Check Constraint between Tables (or change DDL)

check-constraintspostgresql

Given the following PostgreSQL table definitions:

postgres=# create table if not exists messages(
postgres(#     id serial primary key, 
postgres(#     max_length smallint not null check (max_length > 0),
postgres(#     default_text text check (length(default_text) <= max_length),
postgres(#     created_on timestamp with time zone not null
postgres(# );
CREATE TABLE
postgres=# 
postgres=# create table if not exists customized_messages(
postgres(#     message_id serial references messages(id) not null,
postgres(#     message text not null, 
postgres(#     created_on timestamp with time zone not null
postgres(# );
CREATE TABLE

how can I enforce length(customized_messages.text) is <= messages.max_length per the foreign key constraint?

Note that, as-is, I can do the following:

postgres=# insert into messages (id, max_length, default_text, created_on) values (1, 5, null, current_timestamp);
INSERT 0 1
postgres=# insert into customized_messages (message_id, message, created_on)  values (1, 'longer than 5', current_timestamp);
INSERT 0 1
postgres=# select * from messages;
 id | max_length | default_text |          created_on           
----+------------+--------------+-------------------------------
  1 |          5 |              | 2019-01-18 14:12:59.692075-05
(1 row)
postgres=# select * from customized_messages;
 message_id |    message    |          created_on           
------------+---------------+-------------------------------
          1 | longer than 5 | 2019-01-18 14:13:38.226987-05
(1 row)

So, I would like to not allow, e.g. check constraint or something else, the longer than 5 to be inserted since its length is > 5.

Best Answer

One way is to duplicate the max_length column and enforce that they have identical values in the two tables with a composite foreign key (instead of the simple one):

create table if not exists messages(
    id serial
        primary key, 
    max_length smallint not null
        check (max_length > 0),
    default_text text
        check (length(default_text) <= max_length),
    created_on timestamp with time zone not null,
    unique (id, max_length)
);


create table if not exists customized_messages(
    message_id int not null,
    max_length smallint not null,
    message text not null
        check (length(message_text) <= max_length), 
    created_on timestamp with time zone not null,
    foreign key (id, max_length)
        references messages (id, max_length)
);

Note (irrelevant to the question): the message_id column in the 2nd table does not make sense to be serial and at the same time foreign key to the 1st table.

Related Solutions

Postgresql – Postgres multiple joins slow query, how to store default child record

You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

Postgresql – check key if exists in other table without fk constraint

I do agree with Aaron, that not creating a PK (or unique constraint) is a very strange requirement

But you could do something like this:

insert into b (id_b, id_a)
select 1, 2 
where exists (select 1 from a where id_a = 2);

or alternatively:

insert into b (id_b, id_a)
select 1, id_a 
from a where id_a = 2

If you want to insert multiple rows, you can use a values clause:

insert into b (id_b, id_a)
select * 
from (
   values 
      (1,2), (2,3), (3,4)
) as t(id_b, id_a) 
where exists (select 1  
              from a 
              where a.id_a = t.id_a);

This will only make sure that this specific insert will insert correct data. It will not prevent others that don't follow this pattern to insert invalid data. And it is not safe in a multi-user environment where different transactions run the inserts concurrently.

Best Answer

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pgAdmin timing

Postgresql – check key if exists in other table without fk constraint

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