sqlite: insert or update a row, performance issue - sqlite

Question

sqlite table:
CREATE TABLE IF NOT EXISTS INFO
(
uri TEXT PRIMARY KEY,
cap INTEGER,
/* some other columns */
uid TEXT
);
INFO table has 5K+ rows and is run on a low power device (comparable to a 3 year old mobile phone).
I have this task: insert new URI with some values into INFO table, however, if URI is already present, then I need to update uid text field by appending extra text to it if the extra text to be appended isn't found within existing uid string; all other fields should remain unchanged.
As an example: INFO already has uri="http://example.com" with this uid string: "|123||abc||xxx|".
I need to add uri="http://example.com" and uid="|abc|". Since "|abc|" is a substring within existing field for the uri, then nothing should be updated. In any case, remaining fields shouldn't be updated
To get it done I have these options:
build some sql query (if it's possible to do something like that with sqlite in one sql statement),
Do everything manually in two steps: a) retrieve row for uid, do all processing manually and b) update existing or insert a new row if needed
Considering this is constrained device, which way is preferable? What if I omit the the extra requirement of sub-string match and always append uid to existing uid field?

Answer 1

"If it is possible with sqlite in one sql statement":
Yes, it is possible. The "UPSERT" statement has been nicely discussed in this question.
Applied to your extended case, you could do it like this in one statement:
insert or replace into info (uri, cap, uid)
values ( 'http://example.com',
coalesce((select cap from info where uri = 'http://example.com'),'new cap'),
(select case
when (select uid
from info
where uri = 'http://example.com') is null
then '|abc|'
when instr( (select uid
from info
where uri = 'http://example.com'),'|abc|') = 0
then (select uid
from info
where uri = 'http://example.com') || '|abc|'
else (select uid
from info
where uri = 'http://example.com')
end )
);
Checking the EXPLAIN QUERY PLAN gives us
selectid order from detail
---------- ---------- ---------- -------------------------
0 0 0 EXECUTE SCALAR SUBQUERY 0
0 0 0 SEARCH TABLE info USING INDEX sqlite_autoindex_INFO_1 (uri=?)
0 0 0 EXECUTE SCALAR SUBQUERY 1
1 0 0 EXECUTE SCALAR SUBQUERY 2
2 0 0 SEARCH TABLE info USING INDEX sqlite_autoindex_INFO_1 (uri=?)
1 0 0 EXECUTE SCALAR SUBQUERY 3
3 0 0 SEARCH TABLE info USING INDEX sqlite_autoindex_INFO_1 (uri=?)
1 0 0 EXECUTE SCALAR SUBQUERY 4
4 0 0 SEARCH TABLE info USING INDEX sqlite_autoindex_INFO_1 (uri=?)
1 0 0 EXECUTE SCALAR SUBQUERY 5
5 0 0 SEARCH TABLE info USING INDEX sqlite_autoindex_INFO_1 (uri=?)
As far as I know, sqlite will not cache the results of scalar sub-queries (I could not find any evidence of chaching when looking at the VM code produced by EXPLAIN for the above statement). Hence, since sqlite is an in-process db, doing things with two separate statements will most likely perform better than this.
You might want to benchmark the runtimes for this - results will of course depend on your host language and the interface you use (C, JDBC, ODBC, ...).
EDIT
A little performance benchmark using the JDBC driver and sqlite 3.7.2, running 100.000 modifications on a base data set of 5000 rows in table info, 50% updates, 50% inserts, confirms the above conclusion:
Using three prepared statements (first a select, then followed by an update or insert, depending on the selected data): 702ms
Using the above combined statement: 1802ms
The runtimes are quite stable across several runs.