I have general question about FOREIGN KEYs:
- Suppose I have table A with primary key X, and another table B with field Y.
- When I 'ALTER TABLE "B" ADD FOREIGN KEY( "Y" ) REFERENCES "A" ON UPDATE CASCADE ON DELETE CASCADE', that clearly spends some time building a separate index. Since there is already a unique index on X, presumably (?) the index being built is on Y.
- However, the PostgreSQL documentation seems to indicate that it's a good idea to also separately create an index on Y.
- Why, and why is the FOREIGN KEY index different from the ones on X and Y in any way but trivial?
- If I need the separate index on Y, should it be built before or after the FOREIGN KEY constraint?
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