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The check go through the following steps: 1. Run check on the underlying engine. If not ok, then return. 2. Check that there's only one row in the table, and 2.1 warn if more than one row 2.2 return HA_ADMIN_CORRUPT if fewer than one row (i.e. 0 rows) 3. If the sequence is not initialised (e.g. after an ALTER TABLE ... SEQUENCE=1), initialise the sequence by reading the sequence metadata from the table. This will also flush the next_free_value, i.e. set it to the next not cached value (SEQUENCE::reserved_until) 4. Check that the sequence metadata is valid, i.e. nothing out of order e.g. minvalue < maxvalue etc. If invalid it reports HA_ERR_SEQUENCE_INVALID_DATA 5. Check that the sequence has not been exhausted. It reports ER_SEQUENCE_RUN_OUT as a warning if and only if a SELECT NEXTVAL would do so Limitations: 1. The check is independent of flags, so the vanilla check is the same as CHECK ... EXTENDED or CHECK ... FOR UPGRADE etc. 2. When the check discovers invalid metadata from the table, subsequent SELECT NEXTVAL will carry on (or fail) without this piece of knowledge, independent of the CHECK. This is to ensure consistency, i.e. CHECK does not modify behaviour of SELECT, and if anything it makes more sense that SELECT reports HA_ERR_SEQUENCE_INVALID_DATA in this case, regardless of prior CHECK