PostgreSQL: Foreign key on string column

disk-spaceforeign keypostgresqlpostgresql-9.3

I'm currently in a process of redesigning a legacy database. One of the tables primary key is a varchar(254). I need to create a table relating to it.

So for example:

create table "Item" 
(
    "Name" VARCHAR(254) PRIMARY KEY
);

CREATE TABLE "Uses" --new table
(
    id SERIAL PRIMARY KEY NOT NULL,
    "itemName" VARCHAR(254) NOT NULL,
    FOREIGN KEY ("itemName") REFERENCES "Item" ("Name")
);

I was wondering, if each row of Uses will have a reference(memory address) to the Item tables column – that way not copying the value around.

Or will it indeed copy that value, causing superfluous space loss.

Also would like a general opinion on using a VARCHAR as a primary and foreign key. Should maybe better alter the old database to give it a proper id?

Thank you very much.

Best Answer

varchar keys are okay, but they do have some issues:

varchars use more space than ints
you are more likely to have to update a varchar value than an int value, causing cascading updates
might not be appropriate in internationalized applications (i.e. different values for different languages)

To answer your particular question, no there won't be a pointer to the value, the actual value will be stored.

I rarely use varchars in key columns. Sometimes I'll use short char columns like ISO country codes, for convenience.

Also, you can just use the text type in PostgreSQL, as varchar uses text internally. Also, I'd recommend using unquoted snake_case column names.

Related Solutions

Sql-server – How to check the integrity of disabled foreign key relationships for each individual table? (SQL Server 2008)

These DBCC CHECKCONSTRAINTS commands can be run as follows:

DBCC CHECKCONSTRAINTS (TableName) - checks an individual table
DBCC CHECKCONSTRAINTS (ConstraintName) - checks an individual constraint
DBCC CHECKCONSTRAINTS WITH ALL_CONSTRAINTS - checks all constraints in the database
DBCC CHECKCONSTRAINTS WITH ALL_ERRORMSGS - returns all rows that violate constraints
DBCC CHECKCONSTRAINTS WITH NO_INFOMSGS - suppress messages when query runs
DBCC CHECKCONSTRAINTS check only enabled constraints

Keep in mind that SQL Server needs to read through all of your data to check for the constraints, so be careful not to run this across the board for your large or very busy databases. This should be something that is run off hours and also for very large databases you should run this either at a table level or a constraint level.

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

Related Solutions

Sql-server – How to check the integrity of disabled foreign key relationships for each individual table? (SQL Server 2008)

PostgreSQL – insert/update violates foreign key constraints

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