7.6.
LIMIT
and
OFFSET
7.6.
LIMIT
和
OFFSET
LIMIT
and
OFFSET
allow you to retrieve just a portion of the rows that are generated by the rest
of the query:
LIMIT和OFFSET允许只检索查询生成的部分行：
SELECT select_list
FROM table_expression
[ ORDER BY ... ]
[ LIMIT { number | ALL } ] [ OFFSET number ]
If a limit count is given, no more than that many rows will be returned (but possibly fewer, if the
query itself yields fewer rows).
LIMIT ALL
is the same as omitting the
LIMIT
clause, as is
LIMIT
with a NULL argument.
如果限制了输出数量，那么返回行不会多于该限制（但当查询返回行比限制少时，则可能少于该限制）。
LIMIT ALL与省略LIMIT子句相同，类似于LIMIT加NULL参数。
OFFSET
says to skip that many rows before beginning to return rows.
OFFSET 0
is the same as
omitting the
OFFSET
clause, as is
OFFSET
with a NULL argument.
OFFSET指定在返回结果行时跳过的行数。OFFECT 0与省略OFFECT相同，类似于OFFSET加NULL参数。
If both
OFFSET
and
LIMIT
appear, then
OFFSET
rows are skipped before starting to count the
LIMIT
rows that are returned.
如果同时指定OFFSET和LIMIT，那么先跳过OFFSET指定的行，然后再返回LIMIT限定的行数。
When using
LIMIT
, it is important to use an
ORDER BY
clause that constrains the result rows into a
unique order. Otherwise you will get an unpredictable subset of the query's rows. You might be asking
for the tenth through twentieth rows, but tenth through twentieth in what ordering? The ordering is
unknown, unless you specified
ORDER BY
.
当使用LIMIT时，使用ORDER BY子句控制输出行进行排序是很重要的。
否则，将获得查询行的不可预测的子集。假设您可能想要第十到第二十行，但是以什么顺序查询第十到第二十行呢 除非您指定ORDER BY，否则顺序是未知的。
The query optimizer takes
LIMIT
into account when generating query plans, so you are very likely
to get different plans (yielding different row orders) depending on what you give for
LIMIT
and
OFFSET
. Thus, using different
LIMIT
/
OFFSET
values to select different subsets of a query result
will give inconsistent results
unless you enforce a predictable result ordering with
ORDER BY
. This
is not a bug; it is an inherent consequence of the fact that SQL does not promise to deliver the results
of a query in any particular order unless
ORDER BY
is used to constrain the order.
查询优化器在生成查询计划时会考虑LIMIT，因此根据LIMIT和OFFSET的不同，您很可能会获得不同的执行计划（产生不同的行顺序）。因此，除非您使用ORDER BY强制获得可预测的结果排序，否则使用不同的LIMIT / OFFSET值选择查询结果的不同子集将产生不一致的结果。这不是缺陷；因为除非使用ORDER BY约束顺序，否则SQL不会保证以任何特定顺序传递查询结果，这是一个惯例。
The rows skipped by an
OFFSET
clause still have to be computed inside the server; therefore a large
OFFSET
might be inefficient.
使用OFFSET子句跳过的行仍必须在服务器内部进行计算； 因此，较大的OFFSET可能使得效率不高。
