Hi, Slava!
On Jun 12, Slava Akhmechet wrote:
> After some digging I realized that supporting key constraints is a
> responsibility of the storage engine. MySQL defines a lot of
> flags/error modes for this, so I want to clear up my understanding
> before I implement this.
>
> First, there are errors HA_ERR_FOUND_DUPP_KEY,
> HA_ERR_FOUND_DUPP_UNIQUE. From what I understand write_row/update_row
Only HA_ERR_FOUND_DUPP_KEY, you can ignore HA_ERR_FOUND_DUPP_UNIQUE.
> should return these errors if and only if there is a duplicate key or
> a duplicate unique column (respectively), *and* the relevant key has a
> flag HA_NOSAME. Is this correct? Do all keys that must not have
> duplicates have HA_NOSAME flag set?
Yes.
Note that there could be a unique constraint over muptiple columns
(multi-segment key), and some of these columns may allow NULL values.
> Also, how should UNIQUEs be handled? If there is no index on the
> column, write_row would have to scan all the rows to make sure there
> are no duplicates, which seems far too high level for the storage
> engine API. Am I missing something here?
UNIQUE means UNIQUE INDEX, in MySQL there can be no unique constraints
over non-indexed columns.
Regards / Mit vielen Grüßen,
Sergei
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