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LIMIT
In some cases MySQL will handle the query differently when you are
using LIMIT row_count
and not using HAVING
:
LIMIT
, MySQL
will use indexes in some cases when it normally would prefer to do a
full table scan.
LIMIT row_count
with ORDER BY
, MySQL will end the
sorting as soon as it has found the first row_count
lines instead of sorting
the whole table.
LIMIT row_count
with DISTINCT
, MySQL will stop
as soon as it finds row_count
unique rows.
GROUP BY
can be resolved by reading the key in order
(or do a sort on the key) and then calculate summaries until the
key value changes. In this case LIMIT row_count
will not calculate any
unnecessary GROUP BY
values.
#
rows to the client, it
will abort the query (if you are not using SQL_CALC_FOUND_ROWS
).
LIMIT 0
will always quickly return an empty set. This is useful
to check the query and to get the column types of the result columns.
LIMIT row_count
is used to calculate how much space is required.