Update on this:
On Mon, May 5, 2014 at 4:53 PM, Patrick Donnelly <batrick at batbytes.com>
wrote:
> Hi,
>
> I have an INSERT that looks like
>
> INSERT INTO T
> SELECT ...
>
> which I'm running numerous times a second that generally does nothing
> because the SELECT returns no rows. Unfortunately, I've found that
> SQLite still does numerous disk writes anyway in this situation.
>
> Is my only option to eliminate the INSERT by using a SELECT first to
> check if there are no rows? Something like:
>
> CREATE TEMPORARY VIEW V AS
> SELECT ...
> SELECT COUNT(*) FROM V;
> /* If > 0 */
> INSERT INTO T SELECT * FROM V;
>
> ?
I've been able to reproduce it with this minimal example:
CREATE TABLE t1(a INTEGER PRIMARY KEY AUTOINCREMENT);
.print ---------------------------------
INSERT INTO t1
SELECT 0
WHERE 1 = 2;
select changes();
$ ./sqlite3 -vfstrace foo.db < test.sql
...
---------------------------------
trace.xLock(foo.db,SHARED) -> SQLITE_OK
trace.xAccess("/home/batrick/scm/cctools/chirp/src/sqlite-amalgamation-3090200/foo.db-journal",0)
-> SQLITE_OK, out=0
trace.xFileSize(foo.db) -> SQLITE_OK, size=3072
trace.xRead(foo.db,n=16,ofst=24) -> SQLITE_OK
trace.xFileSize(foo.db) -> SQLITE_OK, size=3072
trace.xAccess("/home/batrick/scm/cctools/chirp/src/sqlite-amalgamation-3090200/foo.db-wal",0)
-> SQLITE_OK, out=0
trace.xFileSize(foo.db) -> SQLITE_OK, size=3072
trace.xLock(foo.db,RESERVED) -> SQLITE_OK
trace.xFileControl(foo.db,20) -> SQLITE_OK
trace.xOpen(foo.db-journal,flags=0x806) -> SQLITE_OK
trace.xDeviceCharacteristics(foo.db) -> 0x00001000
trace.xWrite(foo.db-journal,n=512,ofst=0) -> SQLITE_OK
trace.xWrite(foo.db-journal,n=4,ofst=512) -> SQLITE_OK
trace.xWrite(foo.db-journal,n=1024,ofst=516) -> SQLITE_OK
trace.xWrite(foo.db-journal,n=4,ofst=1540) -> SQLITE_OK
trace.xLock(foo.db,EXCLUSIVE) -> SQLITE_OK
trace.xWrite(foo.db-journal,n=4,ofst=1544) -> SQLITE_OK
trace.xWrite(foo.db-journal,n=1024,ofst=1548) -> SQLITE_OK
trace.xWrite(foo.db-journal,n=4,ofst=2572) -> SQLITE_OK
trace.xDeviceCharacteristics(foo.db) -> 0x00001000
trace.xRead(foo.db-journal,n=8,ofst=3072) -> SQLITE_IOERR_SHORT_READ
trace.xSync(foo.db-journal,FULL) -> 0
trace.xWrite(foo.db-journal,n=12,ofst=0) -> SQLITE_OK
trace.xSync(foo.db-journal,FULL) -> 0
trace.xWrite(foo.db,n=1024,ofst=0) -> SQLITE_OK
trace.xWrite(foo.db,n=1024,ofst=2048) -> SQLITE_OK
trace.xFileControl(foo.db,21) -> 12
trace.xSync(foo.db,FULL) -> 0
trace.xClose(foo.db-journal) -> SQLITE_OK
trace.xDelete("/home/batrick/scm/cctools/chirp/src/sqlite-amalgamation-3090200/foo.db-journal",0)
-> SQLITE_OK
trace.xFileControl(foo.db,22) -> 12
trace.xUnlock(foo.db,SHARED) -> SQLITE_OK
trace.xDeviceCharacteristics(foo.db) -> 0x00001000
trace.xUnlock(foo.db,NONE) -> SQLITE_OK
0
trace.xDeviceCharacteristics(foo.db) -> 0x00001000
trace.xUnlock(foo.db,NONE) -> SQLITE_OK
trace.xClose(foo.db) -> SQLITE_OK
No rows were inserted but there are several writes. This behavior
seems to be caused by AUTOINCREMENT?
--
Patrick Donnelly
