Derby
  1. Derby
  2. DERBY-581

Modify SQL to skip N rows of the result and return the next M rows

    Details

    • Type: Improvement Improvement
    • Status: Closed
    • Priority: Minor Minor
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 10.5.1.1
    • Component/s: SQL
    • Labels:
      None
    • Environment:
      All

      Description

      I agree that the information should be expressed in SQL so that the query optimized and execution strategy can know what the user needs in terms of cardinality.

      I'd also like to ask that when we consider extending the SQL in this manner we consider skipping the first N rows and returning the next M rows.

      Craig

      On Sep 20, 2005, at 10:19 AM, Suavi Ali Demir wrote:

      Another little detail about optimization is that Statement.setMaxRows() kind of functions on the JDBC side may not be sufficient since it is called after SQL statement is prepared and returned as an object (after query plan is built). Therefore, it may be necessary to have language syntax to indicate the intention to fetch first 1000 rows only, so that when the query is prepared, this intention can be taken into account.
      Regards,
      Ali

      Mike Matrigali <mikem_app@sbcglobal.net> wrote:
      As craig points out it is important in performance testing to say
      exactly what you are measuring. In general Derby will try to
      stream rows to the user before it has finished looking at all rows.
      So often looking at the first row will and stopping will mean that
      many rows have not been processed. BUT when an order by is involved
      and the query plan either has no appropriate matching index, or decides
      to use a different index then all the rows are processed, then they are
      sent to the sorter and finally after all rows are processed they are
      streamed to the client.

      So as you have seen reading the first 1000 rows of a much larger data
      set can happen very quickly.

      As subsequent mail threads have pointed out, returning the top 1000
      sorted rows is an interesting problem which could be costed and executed
      differently if that information was pushed into the optimizer and the
      sorter (and medium level projects were done in those areas).

      > On Sep 16, 2005, at 4:42 PM, Scott Ogden wrote:
      >
      >
      >
      > I have observed some interesting query performance behavior and am
      > hoping someone here can explain.
      >
      > In my scenario, it appears that an existing index is not being used for
      > the 'order by' part of the operation and as a result the perfo rmance of
      > certain queries is suffering. Can someone explain if this is supposed
      > to be what is happening and why? Please see below for the specific
      > queries and their performance characteristics.
      >
      > Here are the particulars:
      >
      > ---------------------------------
      >
      > create table orders(
      >
      > order_id varchar(50) NOT NULL
      >
      > CONSTRAINT ORDERS_PK PRIMARY KEY,
      >
      > amount numeric(31,2),
      >
      > time date,
      >
      > inv_num varchar(50),
      >
      > line_num varchar(50),
      >
      > phone varchar(50),
      >
      > prod_num varchar(50));
      > > --Load a large amount of data (720,000 records) into the 'orders' table
      > >
      > --Create an index on the time column as that will be used i n the 'where'
      > clause.
      >
      > create index IX_ORDERS_TIME on orders(time);
      > >
      > --When I run a query against this table returning top 1,000 records,
      > this query returns very quickly, consistently less than .010 seconds.
      >>
      >>
      >> select * from orders
      >>
      >> where time > '10/01/2002' and time < '11/30/2002'
      >>
      >> order by time;
      >>
      >> --Now run a similarly query against same table, returning the top
      >> 1,000 records.
      >>
      >> --The difference is that the results are now sorted by the primary key
      >> ('order_id') rather than 'time'.
      >>
      >> --This query returns slowly, approximately 15 seconds. Why??
      >>
      >> select * from orders
      >>
      >> where time > '10/01/2002' and time < '11/30/2002'
      >>
      >> order by order_id;
      >>
      >> --Now run a third query against the same 'orders' table, removing the
      >> where clause
      >>
      >> --This query returns quickly, around .010 seconds.
      >>
      >>
      >>
      >> select * from orders
      >>
      >> order by order_id;
      >>

        Issue Links

        There are no Sub-Tasks for this issue.

          Activity

          Hide
          Jim Newsham added a comment -

          In addition to potential optimization by derby, there are functional reasons why a sql query based method of limiting results are useful (as opposed to at the jdbc level using Statement.setMaxRows()):

          • setMaxRows() can't be used to limit the number of records returned in a subquery
          • if you're generating sql queries programmatically, and don't have access to the Statement object which will perform the query, setMaxRows() can't be used
          Show
          Jim Newsham added a comment - In addition to potential optimization by derby, there are functional reasons why a sql query based method of limiting results are useful (as opposed to at the jdbc level using Statement.setMaxRows()): setMaxRows() can't be used to limit the number of records returned in a subquery if you're generating sql queries programmatically, and don't have access to the Statement object which will perform the query, setMaxRows() can't be used
          Hide
          Dan Scott added a comment -

          There is a standard SQL way of performing the equivalent of LIMIT...OFFSET is through the OLAP window functions – ROW_NUMBER OVER() – that would be the Derby way of limiting results through SQL.

          See http://troels.arvin.dk/db/rdbms/#select-limit for a good description of the SQL standard and comparisons with implementations of this functionality in other databases – but a quick example pasted from that page is:

          SELECT * FROM (
          SELECT
          ROW_NUMBER() OVER (ORDER BY key ASC) AS rownumber,
          columns
          FROM tablename
          ) AS foo
          WHERE rownumber <= n

          Show
          Dan Scott added a comment - There is a standard SQL way of performing the equivalent of LIMIT...OFFSET is through the OLAP window functions – ROW_NUMBER OVER() – that would be the Derby way of limiting results through SQL. See http://troels.arvin.dk/db/rdbms/#select-limit for a good description of the SQL standard and comparisons with implementations of this functionality in other databases – but a quick example pasted from that page is: SELECT * FROM ( SELECT ROW_NUMBER() OVER (ORDER BY key ASC) AS rownumber, columns FROM tablename ) AS foo WHERE rownumber <= n
          Hide
          Bryan Pendleton added a comment -

          I've started a Wiki page to collect information: http://wiki.apache.org/db-derby/OLAPOperations

          Show
          Bryan Pendleton added a comment - I've started a Wiki page to collect information: http://wiki.apache.org/db-derby/OLAPOperations
          Hide
          Bryan Pendleton added a comment -

          I'm considering filing 3 new sub-task JIRA entries, to track the first (of what may be many) sub-projects to investigate specific parts of the OLAP operations features:
          1) null ordering (NULLS FIRST / NULLS LAST in the ORDER BY clause)
          2) GROUPING function (returns 1 if this row is one whose values are the results of aggregation over the specified column reference(s) during a grouped query, and 0 otherwise)
          3) ROW_NUMBER function

          I picked these three as they seemed like incremental, bite-sized features which can be implemented independently and will help us start to get our feet wet with the OLAP operations features.

          Show
          Bryan Pendleton added a comment - I'm considering filing 3 new sub-task JIRA entries, to track the first (of what may be many) sub-projects to investigate specific parts of the OLAP operations features: 1) null ordering (NULLS FIRST / NULLS LAST in the ORDER BY clause) 2) GROUPING function (returns 1 if this row is one whose values are the results of aggregation over the specified column reference(s) during a grouped query, and 0 otherwise) 3) ROW_NUMBER function I picked these three as they seemed like incremental, bite-sized features which can be implemented independently and will help us start to get our feet wet with the OLAP operations features.
          Hide
          Christian d'Heureuse added a comment -

          Bryan, there is already a JIRA entry for NULLS FIRST/LAST: DERBY-2887

          Show
          Christian d'Heureuse added a comment - Bryan, there is already a JIRA entry for NULLS FIRST/LAST: DERBY-2887
          Hide
          Jaco Bester added a comment -

          So, who is in control of this ISSUE!!! Can someone please push this issue to be resolved, PLEASE!!! Great pain to build software solutions for structural weaknesses, which leads to bad software design!!! This issue is now more than 2 years old, and fundamentally one of the most important! Just tell me who I should sleep with

          I would appreciate some ACTION!!!

          Thanx

          Show
          Jaco Bester added a comment - So, who is in control of this ISSUE!!! Can someone please push this issue to be resolved, PLEASE!!! Great pain to build software solutions for structural weaknesses, which leads to bad software design!!! This issue is now more than 2 years old, and fundamentally one of the most important! Just tell me who I should sleep with I would appreciate some ACTION!!! Thanx
          Hide
          Rick Hillegas added a comment -

          Hi Jaco,

          It looks as though Bryan and Thomas are working on this issue through the subtasks listed above: DERBY-2965, DERBY-2998, DERBY-3002, and DERBY-3050. It looks as though the first subtask is finished. I believe that Bryan and Thomas intend to finish the other subtasks for the 10.4 release. See http://wiki.apache.org/db-derby/DerbyTenFourRelease

          Hope this helps,
          -Rick

          Show
          Rick Hillegas added a comment - Hi Jaco, It looks as though Bryan and Thomas are working on this issue through the subtasks listed above: DERBY-2965 , DERBY-2998 , DERBY-3002 , and DERBY-3050 . It looks as though the first subtask is finished. I believe that Bryan and Thomas intend to finish the other subtasks for the 10.4 release. See http://wiki.apache.org/db-derby/DerbyTenFourRelease Hope this helps, -Rick
          Hide
          Aaron Zeckoski added a comment -

          The example shown above does not seem to actually work on Derby 10.4.2:

          This test query works fine (2 results out of 5):
          SELECT * FROM testing WHERE title is not null order by title

          SELECT * FROM (
          SELECT ROW_NUMBER() OVER (ORDER BY title asc) as rownum, columns FROM testing WHERE title is not null
          ) AS tmp WHERE rownum >= 2 and rownum < 4

          Syntax error: Encountered "ORDER" at line 2, column 27.

          Removing the ORDER results in working SQL again:
          SELECT * FROM (
          SELECT ROW_NUMBER() OVER () as rownum, testing.* FROM testing WHERE title is not null
          ) AS tmp WHERE rownum >= 2 and rownum < 4;

          However, attempting to make this work with an order by is hopeless:
          SELECT * FROM (
          SELECT ROW_NUMBER() OVER () as rownum, testing.* FROM testing WHERE title is not null ORDER BY title
          ) AS tmp WHERE rownum >= 2 and rownum < 4

          Syntax error: Encountered "ORDER" at line 2, column 87.

          Is there actually a way to order and limit at the same time? There does not seem to be.

          Show
          Aaron Zeckoski added a comment - The example shown above does not seem to actually work on Derby 10.4.2: This test query works fine (2 results out of 5): SELECT * FROM testing WHERE title is not null order by title SELECT * FROM ( SELECT ROW_NUMBER() OVER (ORDER BY title asc) as rownum, columns FROM testing WHERE title is not null ) AS tmp WHERE rownum >= 2 and rownum < 4 Syntax error: Encountered "ORDER" at line 2, column 27. Removing the ORDER results in working SQL again: SELECT * FROM ( SELECT ROW_NUMBER() OVER () as rownum, testing.* FROM testing WHERE title is not null ) AS tmp WHERE rownum >= 2 and rownum < 4; However, attempting to make this work with an order by is hopeless: SELECT * FROM ( SELECT ROW_NUMBER() OVER () as rownum, testing.* FROM testing WHERE title is not null ORDER BY title ) AS tmp WHERE rownum >= 2 and rownum < 4 Syntax error: Encountered "ORDER" at line 2, column 87. Is there actually a way to order and limit at the same time? There does not seem to be.
          Hide
          Knut Anders Hatlen added a comment -

          > Is there actually a way to order and limit at the same time? There does not seem to be.

          I'm afraid there's no way to do that currently. The implementation of ROW_NUMBER in 10.4 only supports an empty window clause, so you cannot specify OVER (ORDER BY title ASC) yet. Also, ORDER BY in nested SELECT statements is not allowed by the SQL standard, so no help there.

          Show
          Knut Anders Hatlen added a comment - > Is there actually a way to order and limit at the same time? There does not seem to be. I'm afraid there's no way to do that currently. The implementation of ROW_NUMBER in 10.4 only supports an empty window clause, so you cannot specify OVER (ORDER BY title ASC) yet. Also, ORDER BY in nested SELECT statements is not allowed by the SQL standard, so no help there.
          Hide
          A B added a comment -

          > Is there actually a way to order and limit at the same time? There does not seem to be.
          >> I'm afraid there's no way to do that currently.

          Well, there is one way to do this, at least for simple queries, though it's far from intuitive. I don't know if this really usable for your environment, but note the following quote from a comment on DERBY-2998:

          "[The rows returned from the subquery have no guaranteed ordering (Derby doesn't allow ORDER BY in subqueries), and thus any predicate which restricts based on row_number() will restrict the rows based on an undefined order. Since the order of the rows from the subquery may depend on the presence of indexes, the set of rows which survives a row_order()based restriction may depend on the indexes, as well. In the end I do not think this is a bug-but it does strike me as a probable point of confusion for users. It seems that anyone who wants "the first x rows only" has to either accept the fact that "first" does not imply "ordered" (and thus results can vary depending on what conglomerate the optimizer chooses), or else s/he has to use optimizer ovverides to force the optimizer to use an index which is ordered on the desired columns."

          So given that, I think you could try something like:

          create table testing (i int, title varchar(40), author varchar(20));
          insert into testing values (5, 'title_5', 'author_5');
          insert into testing values (4, 'title_4', 'author_4');
          insert into testing values (1, 'title_1', 'inigo');
          insert into testing values (3, 'title_3', 'author_3');
          insert into testing values (2, 'title_2', 'montoya');

          – Your original query; subquery results aren't ordered so you don't know
          – which rows you'll actually get back.

          SELECT * FROM (
          SELECT ROW_NUMBER() OVER () as rownum, testing.*
          FROM testing
          WHERE title is not null
          ) AS tmp
          WHERE rownum >= 2 and rownum < 4;

          ROWNUM |I |TITLE |AUTHOR
          -----------------------------------------------------------------------------
          2 |4 |title_4 |author_4
          3 |1 |title_1 |inigo

          – Create an index whose ordering matches the ordering you want for your subquery.
          create index ix_title_asc on testing(title asc);

          – Now use optimizer overrides to force the index for the subquery. That will force
          – the subquery results to come back in index order, which then means your limit
          – using row_number gives you predictable results.

          SELECT * FROM (
          SELECT ROW_NUMBER() OVER () as rownum, testing.*
          FROM testing --DERBY-PROPERTIES index=ix_title_asc
          WHERE title is not null
          ) AS tmp
          WHERE rownum >= 2 and rownum < 4;

          ROWNUM |I |TITLE |AUTHOR
          -----------------------------------------------------------------------------
          2 |2 |title_2 |montoya
          3 |3 |title_3 |author_3

          As I said, it's not at all intuitive and it's probably not a viable option for complicated queries. But I thought I'd post it just in case it proves useful...

          Show
          A B added a comment - > Is there actually a way to order and limit at the same time? There does not seem to be. >> I'm afraid there's no way to do that currently. Well, there is one way to do this, at least for simple queries, though it's far from intuitive. I don't know if this really usable for your environment, but note the following quote from a comment on DERBY-2998 : "[The rows returned from the subquery have no guaranteed ordering (Derby doesn't allow ORDER BY in subqueries), and thus any predicate which restricts based on row_number() will restrict the rows based on an undefined order. Since the order of the rows from the subquery may depend on the presence of indexes, the set of rows which survives a row_order() based restriction may depend on the indexes, as well. In the end I do not think this is a bug -but it does strike me as a probable point of confusion for users. It seems that anyone who wants "the first x rows only" has to either accept the fact that "first" does not imply "ordered" (and thus results can vary depending on what conglomerate the optimizer chooses), or else s/he has to use optimizer ovverides to force the optimizer to use an index which is ordered on the desired columns." So given that, I think you could try something like: create table testing (i int, title varchar(40), author varchar(20)); insert into testing values (5, 'title_5', 'author_5'); insert into testing values (4, 'title_4', 'author_4'); insert into testing values (1, 'title_1', 'inigo'); insert into testing values (3, 'title_3', 'author_3'); insert into testing values (2, 'title_2', 'montoya'); – Your original query; subquery results aren't ordered so you don't know – which rows you'll actually get back. SELECT * FROM ( SELECT ROW_NUMBER() OVER () as rownum, testing.* FROM testing WHERE title is not null ) AS tmp WHERE rownum >= 2 and rownum < 4; ROWNUM |I |TITLE |AUTHOR ----------------------------------------------------------------------------- 2 |4 |title_4 |author_4 3 |1 |title_1 |inigo – Create an index whose ordering matches the ordering you want for your subquery. create index ix_title_asc on testing(title asc); – Now use optimizer overrides to force the index for the subquery. That will force – the subquery results to come back in index order, which then means your limit – using row_number gives you predictable results. SELECT * FROM ( SELECT ROW_NUMBER() OVER () as rownum, testing.* FROM testing --DERBY-PROPERTIES index=ix_title_asc WHERE title is not null ) AS tmp WHERE rownum >= 2 and rownum < 4; ROWNUM |I |TITLE |AUTHOR ----------------------------------------------------------------------------- 2 |2 |title_2 |montoya 3 |3 |title_3 |author_3 As I said, it's not at all intuitive and it's probably not a viable option for complicated queries. But I thought I'd post it just in case it proves useful...
          Hide
          Knut Anders Hatlen added a comment -

          Since all sub-tasks of this issue have been marked as resolved, and since DERBY-4079 introduced OFFSET/FETCH which provides the functionality requested here, I suggest that we mark this issue as resolved. Any objections?

          Show
          Knut Anders Hatlen added a comment - Since all sub-tasks of this issue have been marked as resolved, and since DERBY-4079 introduced OFFSET/FETCH which provides the functionality requested here, I suggest that we mark this issue as resolved. Any objections?
          Hide
          Rick Hillegas added a comment -

          Hi Knut,

          I think there's more work which could be done to improve our FETCH/OFFSET support (such as adding ORDER BY to subselects), but I agree that this particular issue can be closed. Thanks.

          Show
          Rick Hillegas added a comment - Hi Knut, I think there's more work which could be done to improve our FETCH/OFFSET support (such as adding ORDER BY to subselects), but I agree that this particular issue can be closed. Thanks.
          Hide
          Knut Anders Hatlen added a comment -

          Resolving the issue with fix-version 10.5.0.0.

          Show
          Knut Anders Hatlen added a comment - Resolving the issue with fix-version 10.5.0.0.

            People

            • Assignee:
              Bryan Pendleton
              Reporter:
              Craig L Russell
            • Votes:
              14 Vote for this issue
              Watchers:
              8 Start watching this issue

              Dates

              • Created:
                Updated:
                Resolved:

                Development