7.6. LIMIT and OFFSET

7.6. LIMIT and OFFSET

7.6. LIMIT   and   OFFSET

LIMIT and OFFSET allow you to retrieve just a portion of the rows that are generated by the rest  of the query:

LIMIT and OFFSET allow to retrieve only part of the line generated by the query:

 

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.

If you limit the number of output, then the return line is not more than the limit (but query returns rows than the limit is low, it may be less than the limit). ALL LIMIT LIMIT clause omitted with the same, similar LIMIT plus NULL parameter.

 

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.

Specifies the number of rows OFFSET return result rows skipped. OFFECT 0 OFFECT omitted and the same, similar OFFSET add parameter NULL.

 

If both OFFSET and LIMIT appear, then OFFSET rows are skipped before starting to count the  LIMIT rows that are returned.

Meanwhile, if the specified OFFSET and LIMIT, then skip OFFSET specified row, and then returns the number of rows is defined LIMIT.

 

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 .

When using LIMIT, ORDER BY clause is the control output line is very important to sort. Otherwise, you get unpredictable subset of the query line. Let's say you may want to tenth to the twentieth row, but in what order it queries the tenth to the twentieth row unless you specify ORDER BY, otherwise the order is unknown.

 

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.

When the query optimizer generates a query plan will consider LIMIT, thus depending on the LIMIT and OFFSET, you are likely to get a different execution plan (to produce different row order). So, unless you use ORDER BY forced to get a predictable result ordering, otherwise use different subsets of different LIMIT / OFFSET values ​​to select query result will produce inconsistent results. This is not a defect; because unless you use ORDER BY restraint order, or SQL query results are not guaranteed to be delivered in any particular order, this is a convention.

 

The rows skipped by an OFFSET clause still have to be computed inside the server; therefore a large  OFFSET might be inefficient.

Using rows skipped OFFSET clause still inside the server must be calculated; therefore, such a large OFFSET can be inefficient.