Explanation: Because there is no way to uniquely identify each row in this table, queries and database recovery operations may be slow.

Four different flavors of this message may be produced, flagged as Note, Warning, Caution and Danger:

Note: The table ... has no primary key, unique constraint or unique index.

This table currently has fewer than 1,000 rows so there may be no performance problems, but if the table grows larger those operations may become extremely slow.

Warning: The table ... has no primary key, unique constraint or unique index.

This table currently has more than 1,000 rows; performance may not be a problem now but if the table grows larger those operations may become extremely slow.

Caution: The table ... has no primary key, unique constraint or unique index.

This table has more than 100,000 rows so there is a very real chance those operations will be extremely slow.

Danger: The table ... has no primary key, unique constraint or unique index.

This table has more than one million rows so there is a very real danger those operations will be extremely slow; think geological time scale :)

See also...
The table ... has no primary key or unique constraint, but does have a unique index.