Accelerate SQLite Query - performance

Question

I'm currently learning SQLite (called by Python).
According to my previous question (Reorganising Data in SQLLIte), I want to store multiple time series (Training data) in my database.
I have defined the following fields:
CREATE TABLE VARLIST
(
VarID INTEGER PRIMARY KEY AUTOINCREMENT,
name TEXT UNIQUE NOT NULL
)
CREATE TABLE DATAPOINTS
(
DataID INTEGER PRIMARY KEY,
timeID INTEGER,
VarID INTEGER,
value REAL
)
CREATE TABLE TIMESTAMPS
(
timeID INTEGER PRIMARY KEY AUTOINCREMENT,
TRAININGS_ID INT,
TRAINING_TIME_SECONDS FLOAT
)
VARLIST has 8 entries, TIMESTAMPS 1e5 entries and DATAPOINTS around 5e6.
When I now want to extract data for a given TrainingsID and VarID, I try it like:
SELECT
(SELECT TIMESTAMPS.TRAINING_TIME_SECONDS
FROM TIMESTAMPS
WHERE t.timeID = timeID) AS TRAINING_TIME_SECONDS,
(SELECT value
FROM DATAPOINTS
WHERE DATAPOINTS.timeID = t.timeID and DATAPOINTS.VarID = 2) as value
FROM
(SELECT timeID
FROM TIMESTAMPS
WHERE TRAININGS_ID = 96) as t;
The command EXPLAIN QUERY PLAN delivers:
0|0|0|SCAN TABLE TIMESTAMPS
0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 1
1|0|0|SEARCH TABLE TIMESTAMPS USING INTEGER PRIMARY KEY (rowid=?)
0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 2
2|0|0|SCAN TABLE DATAPOINTS
This basically works.
But there are two problems:
Minor problem: If there is a timeID where no data for the requested VarID is availabe, I get an line with the valueNone`.
I would prefer this line to be skipped.
Big problem: the search is incredibly slow (approx 5 minutes using http://sqlitebrowser.org/).
How do I best improve the performance?
Are there better ways to formulate the SELECT command, or should I modify the database structure itself?

Answer 1

Ok, based on the hints I have got I could extremly accelerate the search by applieng INDEXES as:
CREATE INDEX IF NOT EXISTS DP_Index on DATAPOINTS (VarID,timeID,DataID);
CREATE INDEX IF NOT EXISTS TS_Index on TIMESTAMPS(TRAININGS_ID,timeID);
The EXPLAIN QUERY PLAN output now reads as:
0|0|0|SEARCH TABLE TIMESTAMPS USING COVERING INDEX TS_Index (TRAININGS_ID=?)
0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 1
1|0|0|SEARCH TABLE TIMESTAMPS USING INTEGER PRIMARY KEY (rowid=?)
0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 2
2|0|0|SEARCH TABLE DATAPOINTS USING INDEX DP_Index (VarID=? AND timeID=?)
Thanks for your comments.