Faster Sqlite insert from another table - sql

Question

I have an Sqlite DB which I am doing updates on and its very slow. I am wondering if I am doing it the best way or is there a faster way. My tables are:
create table files(
fileid integer PRIMARY KEY,
name TEXT not null,
sha256 TEXT,
created INT,
mtime INT,
inode INT,
nlink INT,
fsno INT,
sha_id INT,
size INT not null
);
create table fls2 (
fileid integer PRIMARY KEY,
name TEXT not null UNIQUE,
size INT not null,
sha256 TEXT not null,
fs2,
fs3,
fs4,
fs7
);
Table 'files' is actually in an attached DB named ttb. I am then doing this:
UPDATE fls2
SET fs3 = (
SELECT inode || 'X' || mtime || 'X' || nlink
FROM
ttb.files
WHERE
ttb.files.fsno = 3
AND
fls2.name = ttb.files.name
AND
fls2.sha256 = ttb.files.sha256
);
So the idea is, fls2 has values in 'name' which are also present in ttb.files.name. In ttb.files there are other parameters which I want to insert into the corresponding rows in fls2. The query works but I assume the matching up of the two tables is taking the time, and I wonder if theres a more efficient way to do it. There are indexes on each column in fls2 but none on files. I am doing it as a transaction, and pragma journal = memory (although sqlite seems to be ignoring that because a journal file is being created).
It seems slow, so far about 90 minutes for around a million rows in each table.
One CPU is pegged so I assume its not disk bound.
Can anyone suggest a better way to structure the query?
EDIT: EXPLAIN QUERY PLAN
|--SCAN TABLE fls2
`--CORRELATED SCALAR SUBQUERY 1
`--SCAN TABLE files
Not sure what that means though. It carries out the SCAN TABLE files for each SCAN TABLE fls2 hit?
EDIT2:
Well blimey, Crtl-C the query which had been running 2.5 hours at that point, exit Sqlite, run sqlite with the files DB, create index (sha256, name) - 1 minute or so. Exit that, run Sqlite with the main DB. Explain shows that now the latter scan is done with the index. Run the update - takes 150 seconds. Compared to >150 minutes, thats a heck of a speed up. Thanks for the assistance.
TIA, Pete

Answer 1

There are indexes on each column in fls2
Indexes are used for faster selection. They slow down inserts and updates. Maybe removing the one for fls2.fs3 helps?

Answer 2

Not an expert on sqlite, but on some databases it is more performant to insert the data into temporary table, delete them, then insert them from the temp table.
Insert into tmptab
Select fileid,
name,
size,
sha256,
fs2,
inode || 'X' || mtime || 'X' || nlink,
fs4,
fs7
From fls2
Inner join files on
fls2.name = ttb.files.name
AND
fls2.sha256 = ttb.files.sha256
delete from
Fls2 where exists (select 1 from tmptab where tmptab.<primary key> = fls2.<primary key>)
Insert into fls2 select * from tmptab