Question

I have a table (MySQL) that has a column called binID. The values in this column can range from 1 to 70.

What I want to do is select the unique values of this column (should be the numbers from 1 to 70), then iterate over them using each (lets call it theBinID) as a parameter into another SELECT statement such as:

SELECT * FROM MyTable WHERE binID = theBinID ORDER BY createdDate DESC LIMIT 10

Basically, I am looking to get the 10 most recent rows for each binID.

I do not believe there is a way to do this with a basic SQL statement, although I would love that to be the answer, so I have written a stored procedure that creates a cursor on the SELECT DISTINCT of binIDs, and then iterates over it and populates a temp table.

My problem is, this is for an optimization, and if I fetch 100K rows I get 1.7 second average time. Executing my stored procedure to get 700 rows (10 records for 70 bins) is taking 1.4 seconds. I realize that 0.3 seconds could be perceived as considerable improvement, but I am hoping to get this sub-second for 100K rows.

Is there a better way??

The full stored proc is this:

BEGIN
DECLARE done INT DEFAULT FALSE;
DECLARE binID INT;
DECLARE cur1 CURSOR FOR SELECT DISTINCT heatmapBinID from MEStressTest ORDER BY heatmapBinID ASC;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

DROP TEMPORARY TABLE IF EXISTS TempResults;

CREATE TEMPORARY TABLE TempResults (
    `recordID` text NOT NULL,
    `queryTerm` text NOT NULL,
    `recordCreated` double(11,0) NOT NULL,
    `recordByID` text NOT NULL,
    `recordByName` text NOT NULL,
    `recordText` text NOT NULL,
    `recordSource` text NOT NULL,
    `rerecordCount` int(11) NOT NULL DEFAULT '0',
    `timecodeOffset` int(11) NOT NULL DEFAULT '-1',
    `recordByImageURL` text NOT NULL,
    `canDelete` int(11) NOT NULL DEFAULT '1',
    `heatmapBinID` int(11) DEFAULT NULL,
    `timelineBinID` int(11) DEFAULT NULL,
    PRIMARY KEY (`recordID`(20))
);

OPEN cur1;

read_loop: LOOP
    FETCH cur1 INTO binID;

    IF done THEN
        LEAVE read_loop;
    END IF;

    INSERT INTO TempResults (recordID, queryTerm, recordCreated, recordByID, recordByName, recordText, recordSource, rerecordCount, timecodeOffset, recordByImageURL, canDelete, heatmapBinID, timelineBinID)
    SELECT * FROM MEStressTest WHERE heatmapBinID = binID ORDER BY recordCreated DESC LIMIT numRecordsPerBin;
END LOOP;

CLOSE cur1;

SELECT * FROM TempResults ORDER BY heatmapBinID ASC, recordCreated DESC;

END

Answer 1

Try to simulate ROW_NUMBER OVER PARTITION in MySQL: http://www.sqlfiddle.com/#!2/fd8b5/4

Given this data:

create table sentai(
  band varchar(50),
  member_name varchar(50),
  member_year int not null
);

insert into sentai(band, member_name, member_year) values
('BEATLES','JOHN',1960),
('BEATLES','PAUL',1961),
('BEATLES','GEORGE',1962),
('BEATLES','RINGO',1963),
('VOLTES V','STEVE',1970),
('VOLTES V','MARK',1971),
('VOLTES V','BIG BERT',1972),
('VOLTES V','LITTLE JOHN',1973),
('VOLTES V','JAMIE',1964),
('ERASERHEADS','ELY',1990),
('ERASERHEADS','RAYMUND',1991),
('ERASERHEADS','BUDDY',1992),
('ERASERHEADS','MARCUS',1993);

Object, find all three most recent members on each band.

First we have to put a row_number on each member based on most year(via descending order)

select *,

  @rn := @rn + 1 as rn
from (sentai s, (select @rn := 0) as vars)
order by s.band, s.member_year desc;

Output:

|        BAND | MEMBER_NAME | MEMBER_YEAR | @RN := 0 | RN |
|-------------|-------------|-------------|----------|----|
|     BEATLES |       RINGO |        1963 |        0 |  1 |
|     BEATLES |      GEORGE |        1962 |        0 |  2 |
|     BEATLES |        PAUL |        1961 |        0 |  3 |
|     BEATLES |        JOHN |        1960 |        0 |  4 |
| ERASERHEADS |      MARCUS |        1993 |        0 |  5 |
| ERASERHEADS |       BUDDY |        1992 |        0 |  6 |
| ERASERHEADS |     RAYMUND |        1991 |        0 |  7 |
| ERASERHEADS |         ELY |        1990 |        0 |  8 |
|    VOLTES V | LITTLE JOHN |        1973 |        0 |  9 |
|    VOLTES V |    BIG BERT |        1972 |        0 | 10 |
|    VOLTES V |        MARK |        1971 |        0 | 11 |
|    VOLTES V |       STEVE |        1970 |        0 | 12 |
|    VOLTES V |       JAMIE |        1964 |        0 | 13 |

Then we reset the row number when a member is on different band:

select *,

  @rn := IF(@pg = s.band, @rn + 1, 1) as rn,
  @pg := s.band
from (sentai s, (select @pg := null, @rn := 0) as vars)
order by s.band, s.member_year desc;

Output:

|        BAND | MEMBER_NAME | MEMBER_YEAR | @PG := NULL | @RN := 0 | RN | @PG := S.BAND |
|-------------|-------------|-------------|-------------|----------|----|---------------|
|     BEATLES |       RINGO |        1963 |      (null) |        0 |  1 |       BEATLES |
|     BEATLES |      GEORGE |        1962 |      (null) |        0 |  2 |       BEATLES |
|     BEATLES |        PAUL |        1961 |      (null) |        0 |  3 |       BEATLES |
|     BEATLES |        JOHN |        1960 |      (null) |        0 |  4 |       BEATLES |
| ERASERHEADS |      MARCUS |        1993 |      (null) |        0 |  1 |   ERASERHEADS |
| ERASERHEADS |       BUDDY |        1992 |      (null) |        0 |  2 |   ERASERHEADS |
| ERASERHEADS |     RAYMUND |        1991 |      (null) |        0 |  3 |   ERASERHEADS |
| ERASERHEADS |         ELY |        1990 |      (null) |        0 |  4 |   ERASERHEADS |
|    VOLTES V | LITTLE JOHN |        1973 |      (null) |        0 |  1 |      VOLTES V |
|    VOLTES V |    BIG BERT |        1972 |      (null) |        0 |  2 |      VOLTES V |
|    VOLTES V |        MARK |        1971 |      (null) |        0 |  3 |      VOLTES V |
|    VOLTES V |       STEVE |        1970 |      (null) |        0 |  4 |      VOLTES V |
|    VOLTES V |       JAMIE |        1964 |      (null) |        0 |  5 |      VOLTES V |

Then we select only the most recent three members on each band:

select x.band, x.member_name, x.member_year
from
(
  select *,
    @rn := IF(@pg = s.band, @rn + 1, 1) as rn,
    @pg := s.band
  from (sentai s, (select @pg := null, @rn := 0) as vars)
  order by s.band, s.member_year desc
) as x
where x.rn <= 3
order by x.band, x.member_year desc;

Output:

|        BAND | MEMBER_NAME | MEMBER_YEAR |
|-------------|-------------|-------------|
|     BEATLES |       RINGO |        1963 |
|     BEATLES |      GEORGE |        1962 |
|     BEATLES |        PAUL |        1961 |
| ERASERHEADS |      MARCUS |        1993 |
| ERASERHEADS |       BUDDY |        1992 |
| ERASERHEADS |     RAYMUND |        1991 |
|    VOLTES V | LITTLE JOHN |        1973 |
|    VOLTES V |    BIG BERT |        1972 |
|    VOLTES V |        MARK |        1971 |

While windowing function(e.g. ROW_NUMBER OVER PARTITION) is not yet available on MySQL, just simulate it with variables. Please let us know if this is faster than cursor approach

---

How it looks like on windowing-capable RDBMS: http://www.sqlfiddle.com/#!1/fd8b5/6

with member_recentness as
(
  select row_number() over each_band as recent, *
  from sentai
  window each_band as (partition by band order by member_year desc)
)
select * 
from member_recentness
where recent <= 3;

Output:

| RECENT |        BAND | MEMBER_NAME | MEMBER_YEAR |
|--------|-------------|-------------|-------------|
|      1 |     BEATLES |       RINGO |        1963 |
|      2 |     BEATLES |      GEORGE |        1962 |
|      3 |     BEATLES |        PAUL |        1961 |
|      1 | ERASERHEADS |      MARCUS |        1993 |
|      2 | ERASERHEADS |       BUDDY |        1992 |
|      3 | ERASERHEADS |     RAYMUND |        1991 |
|      1 |    VOLTES V | LITTLE JOHN |        1973 |
|      2 |    VOLTES V |    BIG BERT |        1972 |
|      3 |    VOLTES V |        MARK |        1971 |