Question

Sorry just clearing my questions. Extending this question Optimizing sqlite query

I have a table:

CREATE TABLE IF NOT EXISTS [app_status](
[id] INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL ,
[status] TEXT DEFAULT NULL,
[type] INTEGER
)  

I have two indexes. One on status and another on type. Which query will run faster and why?

SELECT COALESCE(min(type), 0)
  FROM app_status
 WHERE status IS NOT NULL
   AND type IN (1,2) limit 1

QUERY PLAN o/p

0|0|0|SEARCH TABLE app_status USING INDEX idx_type (mailbox_type=?) (~10 rows)
0|0|0|EXECUTE LIST SUBQUERY 1

Or…

SELECT type FROM
app_status WHERE
status IS NOT NULL
ORDER BY type limit 1

QUERY PLAN o/p

0|0|0|SCAN TABLE app_status USING INDEX idx_type (~500000 rows)

Answer 1

The first query returns zero or one row matching the criteria in the WHERE clause (where status is not null and type in (1,2) , in unspecified order.

The second query finds all the rows matching the criteria in the WHERE clause (where status is not null), sorts them by type and then returns zero or 1 row.

You should note that the two queries, while they may return the identical results, are not guaranteed to. In particular, the row returned by the second query will return the first row of the result set as ordered by type, regardless of what value that type is. If the lowest value for type where `status is not null is, say 157, that is the row you are going to get. The first query, in that case, will return 0 rows.

But assuming type and status are indexed and the query can use one or more of the indexes, then my suspicion is the first query would be faster as it can seek directly to the desired row(s).

But much depends on the shape of the data (how much data is there? How is it distributed? etc.), whether or not the index is ‘covering’ (if the index doesn’t cover all the columns in the query, then it must do additional I/O to get the data page(s) required to cover all the columns.

edited to note Looking at the execution plans you posted (not knowing Sqllite), the first plan says it should return about 10 rows; the second about 50,000 rows. Which do you think might be faster?