![]() The LIMIT clause has one more optional syntax provided to maintain compatibility with the PostgreSQL syntax that is as follows. By default, when not specified the type of order, it is considered ascending type. In the ORDER BY clause, we can specify a list of columns on which we want to define the order of the resultset and then mention whether the order should be in an ascending or descending manner using ASC and DESC. In that case, both of the following limit clauses work in the same way: LIMIT 5 īoth of them will retrieve the first 5 records from the select query’s resultset. Whenever we do not specify the offset value, the default value is considered 0, and the row count begins from the 1-row number. If the row_count will have been 5, then beginning from the fourth record, 5 rows would have been retrieved with row numbers 4,5,6,7 and 8. in this case, from the fourth row, four records will be retrieved as the row_count is 4. When the row_count is mentioned as 4, then starting from the defined offset, i.e. If the offset is specified as 3, then the row count will begin from the fourth row as the offset begins from 0 by default. The following figure will help you to understand the offset and row_count concepts clearly –Ĭonsider that the above diagram shows the records of the table, and the numbers in it stand for the row number of that records. The offset is used for specifying the position of the record from where the result set is to be fetched from the table named name_of_table. The offset is the integer value that is optional and has the default value is 0. The count_of_rows is the integer value that helps us to specify the row count up to which the number of the records is to retrieve from the name_of_table table. The syntax of the LIMIT clause, along with the ORDER BY clause and the place where they should be used, is shown below: SELECT ORDER BY clause can consist of ordering the data based on one or more column values ascending or descending. Out of which one is required, that is count_of_rows is required, and the other one named offset is optional. Then the table itself will be involved only for fetching those 200 rows that are to be displayed.The limit clause accepts two arguments. 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.) Or (variation) using the PK as well, with an index on (seriesName, retreivalTime, dbId) and query: SELECT d.dbId, It won't be super fast but probably more efficient than what you have: SELECT d.dbId,ĪND di.max_retreivalTime = d.retreivalTime Here is a suggestion: add an index on (seriesName, retreivalTime) and try this query. 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. Insert performance is not critical here, so if I need to create an additional index or two, that's fine. How can I optimize this query? I can provide the raw query operations if needed. That would have somewhat poor performance for cases where OFFSET is large, but that happens very rarely in this application. 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 Ġ|0|0|SCAN TABLE DataItems USING INDEX DataItems_time_indexīasically, 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. The index on seriesName is COLLATE NOCASE, if that's relevant. However, EXPLAIN QUERY PLAN seems to indicate they're not being used: sqlite> EXPLAIN QUERY PLAN SELECT dbId, ![]() sqlite> SELECT name FROM sqlite_master WHERE type='index' ORDER BY name ĭataItems_time_index // This is the index on retreivalTime. I have indexes on both seriesName and retreivalTime. It takes approximately 800 milliseconds to execute on a table with ~67K rows. Where limit is typically ~200, and offset is 0 (they drive a pagination mechanism).Īnyways, right now, this one query is completely killing my performance. Right now, I am generating some content to display using the following query: SELECT dbId, I have a little web-application that is using sqlite3 as it's DB (the db is fairly small).
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