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I have a tickets table with an id that I need to associate to a lookup table where the counterpart of that data is another id that is controlled from an outside source.

tickets
- id
- sutff

lookup
- outside_data_id
- ticket_id

I just need to know if the association exists (hence the lookup table) and has a one to many relationship. I know nothing else about the outside_data_id except the id itself and the related ticket_id.

Also I don't want to alter the tickets table as this will only affect a small amount of ticket_id's.

Example

outside_data_id = 1234

Can be related to:

ticket_id = 321
ticket_id = 322
ticket_id = 900

Would the below lookup table structure work or is there a better way?

CREATE TABLE lookup
(
   outside_data_id integer, 
   ticket_id integer
)
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I am not completely sure what you are looking up. Do you have an outside_data_id and want to look up related ticket_id's? Or a ticket_id and want to verify it is linked to an outside_data_id? Or both directions? An optimal answer depends on these specifics. Also, is the combined (ticket_id, outside_data_id) unique - so no dupes in table lookup? –  Erwin Brandstetter Oct 2 '12 at 22:50
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3 Answers 3

up vote 4 down vote accepted

Your table will work fine for this purpose, but you probably want to add an index. If the primary reason for using this table is to take an outside_ticket_id and get the corresponding ticket_id's I would add the following clustered index:

CREATE CLUSTERED INDEX [CL_Lookup_OD_ID] on [lookup](outside_data_id)
GO

If the primary lookup will be the other way around (trying to find the outside_data_id from a ticket_id) place the clustered index on the other column.

---- Oh, sorry, just noticed this is Postgres. The above syntax is SQL Server. For Postgres, create an index on the column, then issue the cluster command, like so:

CREATE INDEX IX_outside_data_id on lookup(outside_data_id);
CLUSTER lookup using IX_outside_data_id;

You may also want to look at the "fillfactor" on the index depending on how heavy the insert load is here. But that is a large topic worth exploring on it's own...

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Thanks, was also looking for the INDEX but didn't know about Clusters –  Phill Pafford Oct 2 '12 at 18:42
    
CLUSTER in Postgres does not do the same as a clustered index in SQL server. Not sure what this is supposed to achieve. –  Erwin Brandstetter 35 secs ago
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A structure like yours should probably be solved with:
- a multi-column primary key constraint on the m-table (lookup) and
- a foreign key constraint referencing the primary key of the 1-table.

An optimal index for looking up values in one direction is provided automatically by the primary key of the lookup table.

CREATE TABLE ticket (
   ticket_id integer PRIMARY KEY  -- possibly serial instead of integer
 , stuff text
);

CREATE TABLE lookup (
   outside_data_id integer
 , ticket_id integer REFERENCES ticket(ticket_id)
      -- ON UPDATE CASCADE  -- optional
      -- ON DELETE CASCADE  -- optional
 , CONSTRAINT lookup_pkey PRIMARY KEY (outside_data_id, ticket_id)
);

Never use the non-descriptive column name id. Some not-so-smart ORMs do it, but it's generally unhelpful - an anti-pattern. Use a descriptive, distinct name like ticket_id.

If you ..

just need to know if the association exists

... then a good old foreign key might be all you need. It takes care of that automatically and delivers the additional bonus of relational integrity being enforced no matter what. Plus more options.
It also insists on a related item for each and every filter_id, so you may not be able to use it.

Actual indexes needed depend on your workload, which is still unclear to me after reading your question multiple times. In particular, the order of columns in a multi-column index (or a primary key constraint for that matter) is relevant - as we have discussed in depth under this related question.

For optimal performance (neglecting costs for index maintenance, assuming the table isn't updated much) and if your queries go both ways, you would create another index in addition to the primary key as defined above:

CREATE INDEX lookup_reverse_idx(ticket_id, outside_data_id);

While you could cover most additional use-cases with just a single-column index on ticket_id, due to data alignment in PostgreSQL storage, two integer columns in your index result in the same size on disk as just one. So, hardly any cost for some additional gain.

Aside

Experience gathered with different RDBMS'es is not always applicable across platforms.

PostgreSQL doesn't have a CLUSTERED INDEX like SQL server.

PostgreSQL's CLUSTER command is loosely related, but works differently. It's a one-time operation and does not keep the table clustered. It also completely rewrites the table with all effects of a VACUUM FULL (in modern versions since at least 9.0).

Depending on your actual workload CLUSTER may or may not be useful. It can be for looking up multiple related rows (outside_data_id -> ticket_id), especially for tables that are not updated a lot.

Advice on FILLFACTOR is good, though, especially if you use CLUSTER - which may otherwise actually be hurting performance if you UPDATE a lot.

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Thanks for the clarification and fact-filled answer! –  Max Vernon Oct 2 '12 at 22:43
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The structure you've created seems like a perfectly reasonable lookup table.

CREATE TABLE lookup1
(
   outside_data_id integer NOT NULL 
   , ticket_id integer NOT NULL
);

I would probably add an index to this table like:

CREATE CLUSTERED INDEX IX_Lookup1 
    ON Lookup1 (outside_data_id, ticket_id) 
    WITH (ALLOW_PAGE_LOCKS=ON, ALLOW_ROW_LOCKS=ON, FILLFACTOR=100);

(My CREATE INDEX sample is for SQL Server, most likely there will be some changes needed for PostgreSQL!)

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1  
PostgreSQL doesn't have a CLUSTERED INDEX like in SQL server. There is the loosely related CLUSTER command ... I added a bit to my answer. –  Erwin Brandstetter Oct 2 '12 at 22:38
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