SQLite Optimize Query - sql

Question

I'm having trouble with SQLite query optimization, it runs fine, but for large tables it takes too much time and I need some help with optimizing it.
Source table:
-------+----------+----------------
IdMain | IdParent | ColumnToUpdate
-------+----------+----------------
1 | |
2 | 1 | 999 <-- IdParent = 1 \
3 | | \
4 | 5 | 123 > DISTINCT ITEMS COUNT = 1
5 | | / IdParent = 1
6 | 1 | 999 <-- IdParent = 1 / UPDATE Row with IdMain = IdParent
7 | 4 |
8 | 3 | 456
-------+----------+----------------
Query to optimize
UPDATE Table
SET ColumnToUpdate = (SELECT DISTINCT ColumnToUpdate
FROM Table T
WHERE T.ColumnToUpdate IS NOT NULL
AND T.IdParent = Table.IdMain)
WHERE Table.ColumnToUpdate IS NULL
AND (SELECT COUNT(*) FROM (SELECT DISTINCT ColumnToUpdate
FROM Table T2
WHERE T2.ColumnToUpdate IS NOT NULL
AND T2.IdParent = Table.IdMain)) = 1 ;
Expected table
-------+----------+----------------
IdMain | IdParent | ColumnToUpdate
-------+----------+----------------
1 | | 999 <-- UPDATE
2 | 1 | 999
3 | |
4 | 5 | 123
5 | |
6 | 1 | 999
7 | 4 |
8 | 3 | 456
-------+----------+----------------
Pseudo algorithm
FOR Row DO
BEGIN
IF ColumnToUpdate = NULL THEN
BEGIN
// count distinct values in ColumnToUpdate
X = COUNT(DISTINCT(ColumnToUpdate(WITH IdParent = IdMain))
// update row ONLY when number of distinct count equals = 1
IF X = 1 THEN
UPDATE(ColumnToUpdate)
END
END
I've tried to split it up within the source code (currently Delphi) but it works slow too. Is there any way to speed it up?

Answer 1

I wonder if this might speed things up:
UPDATE Table
SET ColumnToUpdate = coalesce((SELECT ColumnToUpdate
FROM Table T
WHERE T.ColumnToUpdate IS NOT NULL AND
T.IdParent = Table.IdMain
GROUP BY ColumnToUpdate
HAVING count(*) = 1),
Table.ColumnToUpdate
)
WHERE Table.ColumnToUpdate IS NULL;
This only executes the subquery once instead of twice.
Also, an index on Table(IdParent, ColumnToUpdate) might also improve performance.