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DROP COLUMN feature with indexes
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Take the following scenario, where there is no explicit index to drop:

    CREATE TABLE t1(
        a INTEGER primary key,
        b INTEGER,
        c INTEGER,
        UNIQUE(b,c)
    );

    ALTER TABLE t1 DROP COLUMN c;

If the DROP COLUMN fails instead of removing the automatic index, then the caller has no option but to re-create the table completely. This reduces the useful scope of DROP COLUMN.