Improving `GROUP BY` query performance in sqlite3

Question

I have a little web-application that is using sqlite3 as it's DB (the db is fairly small).

Right now, I am generating some content to display using the following query:

SELECT dbId,
        dlState,
        retreivalTime,
        seriesName,
        <snip irrelevant columns>
        FROM DataItems
        GROUP BY seriesName
        ORDER BY retreivalTime DESC
        LIMIT ?
        OFFSET ?;

Where limit is typically ~200, and offset is 0 (they drive a pagination mechanism).

Anyways, right now, this one query is completely killing my performance. It takes approximately 800 milliseconds to execute on a table with ~67K rows.

I have indexes on both seriesName and retreivalTime.

sqlite> SELECT name FROM sqlite_master WHERE type='index' ORDER BY name;
<snip irrelevant indexes>
DataItems_seriesName_index
DataItems_time_index           // This is the index on retreivalTime. Yeah, it's poorly named

However, EXPLAIN QUERY PLAN seems to indicate they're not being used:

sqlite> EXPLAIN QUERY PLAN SELECT dbId, 
                                  dlState, 
                                  retreivalTime, 
                                  seriesName 
                                  FROM 
                                      DataItems 
                                  GROUP BY 
                                      seriesName 
                                  ORDER BY 
                                      retreivalTime 
                                  DESC LIMIT 200 OFFSET 0;
0|0|0|SCAN TABLE DataItems
0|0|0|USE TEMP B-TREE FOR GROUP BY
0|0|0|USE TEMP B-TREE FOR ORDER BY

The index on seriesName is COLLATE NOCASE, if that's relevant.

If I drop the GROUP BY, it behaves as expected:

sqlite> EXPLAIN QUERY PLAN SELECT dbId, dlState, retreivalTime, seriesName FROM DataItems ORDER BY retreivalTime DESC LIMIT 200 OFFSET 0;
0|0|0|SCAN TABLE DataItems USING INDEX DataItems_time_index

Basically, my naive assumption would be that the best way to perform this query would be to walk backwards from latest value in retreivalTime, and every time a new value for seriesName is seen, append it to a temporary list, and finally return that value. That would have somewhat poor performance for cases where OFFSET is large, but that happens very rarely in this application.

How can I optimize this query? I can provide the raw query operations if needed.

Insert performance is not critical here, so if I need to create an additional index or two, that's fine.

My current thoughts are a commit-hook that updates a separate table that is used to track only unique items, but that seems like overkill.

score 5 · Answer 1 · answered Jul 25 '14 at 11:02

An index can be used to optimize the GROUP BY, but if the ORDER BY uses different columns, the sorting cannot use an index (because an index would help only if the database would be able to read the rows from the table in the sort order).

A COLLATE NOCASE index does not help if you use a different collation in the query. Add a 'normal' index, or use GROUP BY seriesName COLLATE NOCASE, if that is allowed.

Using the OFFSET clause for pagination is not very efficient, because the database still has to group and sort all rows before it can begin stepping over them. Better use a scrolling cursor.

Note: there is no guarantee that the dbId and dlState values come from any specific row; SQLite allows non-aggregated columns in an aggregated query only for bug compatibility with MySQL.

ypercubeᵀᴹ · Accepted Answer · 2014-07-25T11:41:20.033

Here is a suggestion: add an index on (seriesName, retreivalTime) and try this query. It won't be super fast but probably more efficient than what you have:

SELECT d.dbId,
       d.dlState,
       d.retreivalTime,
       d.seriesName,
        <snip irrelevant columns>
FROM DataItems AS d
  JOIN
    ( SELECT seriesName, 
             MAX(retreivalTime) AS max_retreivalTime
      FROM DataItems
      GROUP BY seriesName
      ORDER BY max_retreivalTime DESC
      LIMIT ?
      OFFSET ?
    ) AS di
    ON  di.seriesName = d.seriesName
    AND di.max_retreivalTime = d.retreivalTime
ORDER BY di.max_retreivalTime ;

Or (variation) using the PK as well, with an index on (seriesName, retreivalTime, dbId) and query:

SELECT d.dbId,
       d.dlState,
       d.retreivalTime,
       d.seriesName,
        <snip irrelevant columns>
FROM DataItems AS d
  JOIN
    ( SELECT dbId
      FROM DataItems
      GROUP BY seriesName
      ORDER BY MAX(retreivalTime) DESC
      LIMIT ?
      OFFSET ?
    ) AS di
    ON  di.dbId = d.dbId
ORDER BY d.max_retreivalTime ;

The logic behind the queries is to use only the index for the derived table calculations (finding the max(retreival-time) for every seriesName and then order by and do the offset-limit thing.)

Then the table itself will be involved only for fetching those 200 rows that are to be displayed.

Improving `GROUP BY` query performance in sqlite3

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