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Comments on Why would an unique index get moved to the primary key after the underlying column is dropped?

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Why would an unique index get moved to the primary key after the underlying column is dropped?

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So I had a table with a primary key and a bunch of different columns. Columns A, B, and C were all unsigned ints (like the primary key column) and each column had a unique constraint

I dropped the A, B, and C columns, and then I got a warning that there were now duplicate unique constraints. When I checked the table structure I saw that unique indexes had been moved to the primary key column.

My assumption was that dropping a column would also drop its index, why would the index be moved to the primary key?

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I can't say that I fully understood what you did, but I think I got the explanation:

If columns are dropped from a table, the columns are also removed from any index of which they are apart. If all columns that make up an index are dropped, the index is dropped as well.

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General comments (2 comments)
General comments
Charlie Brumbaugh‭ wrote over 3 years ago

Each column had a unique key, not a unique key on the combination

Alexei‭ wrote over 3 years ago

@Charlie Brumbaugh‭ Yes, that means that the index contains that column + columns in the PK. That would explain the second point which actually seems to be the only point: all indexes will only cover the column in the PK.