For "concurrent inserts", MySQL reference manual has the following explanation:
The MyISAM storage engine supports concurrent inserts to reduce contention between readers and writers for a given table: If a MyISAM table has no holes in the data file (deleted rows in the middle), an INSERT statement can be executed to add rows to the end of the table at the same time that SELECT statements are reading rows from the table.
Let's say our database "concurrent insert" parameter is set to "Auto" (1).
And we have a MyISAM table with a gap. When we insert new rows and fill those gaps, does the table "immediately" get ready to accept "concurrent inserts" for future insert queries?
Or do we need to run "OPTIMIZE" before the table knows there are no gaps?