Incorrect value for UNIQUE_CONSTRAINT_NAME in REFERENTIAL_CONSTRAINTS

Question

I am listing all FK constraints for a given table using INFORMATION_SCHEMA set of views with the following query:

SELECT      X.UNIQUE_CONSTRAINT_NAME,
            "C".*, "X".*
FROM        "INFORMATION_SCHEMA"."KEY_COLUMN_USAGE" AS "C"
INNER JOIN  "INFORMATION_SCHEMA"."REFERENTIAL_CONSTRAINTS" AS "X"
        ON  "C"."CONSTRAINT_NAME" = "X"."CONSTRAINT_NAME" 
        AND "C"."TABLE_NAME" = 'MY_TABLE'
        AND "C"."TABLE_SCHEMA" = 'MY_SCHEMA'

Everything works perfectly well, but for one particular constraint the value of UNIQUE_CONSTRAINT_NAME column is wrong, and I need it in order to find additional information from the referenced Column. Basically, for most of the rows the UNIQUE_CONSTRAINT_NAME contains the name of the unique constraint (or PK) in the referenced table, but for one particular FK it is the name of some other unique constraint.

I dropped and re-created the FK - did not help.

My assumption is that the meta-data is somehow screwed. Is there a way to rebuild the meta data so that the INFORMATION_SCHEMA views would actually show the correct data?

edit-1: sample db structure

CREATE TABLE MY_PARENT_TABLE (
    ID      INTEGER,
    NAME    VARCHAR,
    --//...

    CONSTRAINT MY_PARENT_TABLE_PK PRIMARY KEY CLUSTERED (ID)
)

CREATE UNIQUE NONCLUSTERED INDEX MY_PARENT_TABLE_u_nci_ID_LongName ON MY_PARENT_TABLE (ID ASC) INCLUDE (SOME_OTHER_COLUMN)

CREATE TABLE MY_CHILD_TABLE (
    ID      INTEGER,
    PID     INTEGER,
    NAME    VARCHAR,

    CONSTRAINT MY_CHILD_TABLE_PK PRIMARY KEY CLUSTERED (ID)
   ,CONSTRAINT MY_CHILD_TABLE__MY_PARENT_TABLE__FK
        FOREIGN KEY (PID)
        REFERENCES MY_PARENT_TABLE (ID)
        ON UPDATE NO ACTION
        ON DELETE NO ACTION
)

I expect the UNIQUE_CONSTRAINT_NAME to be MY_PARENT_TABLE_PK, but what I am getting is MY_PARENT_TABLE_u_nci_ID_LongName.

Having looked at the structure, I see that in fact there are 2 UNIQUE constaints on that column - PK and the MY_PARENT_TABLE_u_nci_ID_LongName. So the real question should probably be: why does it take some other unique index and not the PK?

Answer 1

Since you have both a PK and a UNIQUE constraint on the same column, SQL Server picks one to use. I don't know if it picks the UNIQUE constraint because it is thinner (i.e. fewer columns involved) and might require fewer reads to confirm matches(?)

I don't see any way within SQL to enforce which one it chooses, other than ordering your scripts - create the table with the PK, create the other table and the FK, then create the UNIQUE constraint if you really need it - but is that really the case?