PostgreSQL – Temp Table Ignores ‘Excluding Constraints’

postgresqltemporary-tables

In PostgreSQL 9.5.

I'd like to create a temporary copy of my table without its primary key. Postgres has a feature { INCLUDING | EXCLUDING } { DEFAULTS | CONSTRAINTS | INDEXES | STORAGE | COMMENTS | ALL } – which I'm attempting to use.

create table test (
    col1    integer,
    col2    integer,
    col3    text,
    primary key (col1)
);

create temporary table t (like test excluding constraints);

insert into t (col1, col2, col3) values (1, 2, 'foo'), (null, 2, 'bar');

If I'm understanding excluding constraints correctly, that should not bring over the non-null constraint on test to t.

But here's the error that I receive:

ERROR:  null value in column "col1" violates not-null constraint
DETAIL:  Failing row contains (null, 2, bar).
********** Error **********

ERROR: null value in column "col1" violates not-null constraint
SQL state: 23502
Detail: Failing row contains (null, 2, bar).

I have a workaround, create temporary table t as select * from test where true = false, but I'd like to know what I'm not understanding correctly about excluding constraints.

Best Answer

I'd like to know what I'm not understanding correctly about excluding constraints.

What is happening here is that primary key (col1) creates a primary key constraint but doesn't create a separate not null constraint. It adds a not null "modifier" to the column. This works as a constraint, ie. no nulls are allowed in col1 but it is not a named constraint. That's why it's not removed by the excluding constraints, it's a column property.

As the CREATE TABLE documentation states:

Not-null constraints are always copied to the new table. CHECK constraints will be copied only if INCLUDING CONSTRAINTS is specified. ..

If you had the table defined with an explicit CHECK column is not null constraint instead:

create table test (
    col1    integer,
    col2    integer,
    col3    text,
    unique (col1),
    check (col1 is not null)
);

then the excluding constraints would have succeeded. But if the table has a primary key defined, then all the columns of this primary key are created with the not null modifier and a create .. like test will copy these modifiers.

Your workaround is valid. Either of these will work:

create temporary table t as select * from test where false ;
create temporary table t as select * from test limit 0 ;

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Postgresql – Violates foreign key constraint

Basically foreign key constraints are not inherited. If you are working with table inheritance you have a few options.

Stop enforcing foreign keys
Use constraint triggers to enforce foreign keys

In most cases you are better off with a single large table and smaller join tables possibly with deferred foreign keys. Unfortunately these are not possible either to fully do the things that table inheritance can do so typically there is going to be at least some custom coding.

PostgreSQL – insert/update violates foreign key constraints

There are a few problems with your tables. I'll try to address the foreign keys first, since you question asked about them :)

But before that, we should realize that the two sets of tables (the first three you created and the second set, which you created after dropping the first set) are the same. Of course, the definition of Table3 in your second attempt has syntax and logical errors, but the basic idea is:

CREATE TABLE table3 (   
  "ID" bigint NOT NULL DEFAULT '0',   
  "DataID" bigint DEFAULT NULL,   
  "Address" numeric(20) DEFAULT NULL,   
  "Data" bigint DEFAULT NULL,
   PRIMARY KEY ("ID"),   
   FOREIGN KEY ("DataID") REFERENCES Table1("DataID") on delete cascade on update cascade,   
   FOREIGN KEY ("Address") REFERENCES Table2("Address") on delete cascade on update cascade
);

This definition tell PostgreSQL roughly the following: "Create a table with four columns, one will be the primary key (PK), the others can be NULL. If a new row is inserted, check DataID and Address: if they contain a non-NULL value (say 27856), then check Table1 for DataID˙and Table2 for Address. If there is no such value in those tables, then return an error." This last point which you've seen first:

ERROR: insert or update on table "Table3" violates foreign key constraint 
    "Table3_DataID_fkey" DETAIL: Key (DataID)=(27856) is not present in table "Table1".

So simple: if there is no row in Table1 where DataID = 27856, then you can't insert that row into Table3.

If you need that row, you should first insert a row into Table1 with DataID = 27856, and only then try to insert into Table3. If this seems to you not what you want, please describe in a few sentences what you want to achieve, and we can help with a good design.

And now about the other problems.

You define your PKs as

CREATE all_your_tables (
    first_column NOT NULL DEFAULT '0',   
    [...]
    PRIMARY KEY ("ID"),

A primary key means that all the items in it are different from each other, that is, the values are UNIQUE. If you give a static DEFAULT (like '0') to a UNIQUE column, you will experience bad surprises all the time. This is what you got in your third error message.

Furthermore, '0' means a text string, but not a number (bigint or numeric in your case). Use simply 0 instead (or don't use it at all, as I written above).

And a last point (I may be wrong here): in Table2, your Address field is set to numeric(20). At the same time, it is the PK of the table. The column name and the data type suggests that this address can change in the future. If this is true, than it is a very bad choice for a PK. Think about the following scenario: you have an address '1234567890454', which has a child in Table3 like

ID        DataID           Address             Data
123       3216547          1234567890454       654897564134569

Now that address happens to change to something other. How do you make your child row in Table3 follow its parent to the new address? (There are solutions for this, but can cause much confusion.) If this is your case, add an ID column to your table, which will not contain any information from the real world, it will simply serve as an identification value (that is, ID) for an address.

Best Answer

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