On 10/11/2013 02:29 PM, Paul Harris wrote:
Hi again,
I am wondering if there is any performance benefit in *not* wrapping
multiple SELECT calls in a BEGIN/END transaction?
As I understand it, with wrapping:
BEGIN -- does nothing (yet)
SELECT1 -- creates a SHARE-ONLY lock on the DB
SELECT2 -- nothing extra
SELECT3 -- nothing extra
END -- cancels SHARE lock
Or, without wrapping, does it create and drop the SHARE LOCK each time?
Does that add overhead? ie:
Yes. Adds the overhead of obtaining and releasing the SHARED lock
each transaction.
SELECT1 -- creates a SHARE-ONLY lock on the DB, selects, then drops lock
SELECT2 -- creates a SHARE-ONLY lock on the DB, selects, then drops lock
SELECT3 -- creates a SHARE-ONLY lock on the DB, selects, then drops lock
Also, silly question, but if I want to find a row and then read a blob
using the sqlite3_blob_open API, I assume I must use transactions to get a
consistent read, right?
ie, without a transaction,
SELECT rowid FROM table WHERE condition;
** another client could make a change to a db here **
sqlite3_blob_open( rowid )
** blob may now be for the "wrong row"
Transaction is required, yes. Otherwise some other connection may
delete the row in question between the SELECT and sqlite3_blob_open()
calls.
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