> 16.03.2018 14:47, shg_siste...@yahoo.com.ar [firebird-support] wrote: >> Since some days ago I'm getting (it seems very occasionally and randomly) a >> "blob not >> found" error. >> As far as I remember the problem happens during read operations: with >> "locate" commands or >> inside a "while not eof" loop...
Dmitry S. wrote > You should use "Repeatable Read" or "Snapshot" transaction isolation mode. > In "Read > Committed" you can get this error if record has been deleted and garbage > collected before > you start reading the BLOB. Also bear in mind that: 1. If no blob has ever been stored for that field in a record, then no blob id exists for it. For editing, you should test for NULL at some appropriate point in your query or your client code and handle it. 2. When you "edit" a blob, the original blob is copied in memory to a new blob with a temporary blob id. The temporary blob is visible only to the transaction in which it is created and, within that transaction, the original blob content is invalidated as garbage by the Post action from the client application. 3. On commit, the temporary blob is copied to a new location, with a new blob id, and becomes visible to other transactions from that point. It will not go to the same physical location as the original blob, because that location remains "interesting" to the garbage collector until it is able to be released by a future GC. 4. Be aware that, if you try to edit the same blob field more than once in the same transaction, the initial view of the original blob is not there any more. AFAIR, the Firebird engine began enforcing read-only on changed records from v.2.5 onward, so you would encounter a different error if your app was trying to post multiple edits of non-blob fields from the same transaction. I can't say for sure whether that restriction could apply to blob edits since the new blob id is not known until the commit has occurred. Also, I don't know whether your IBDAC components are aware of the restriction on multiple edits. It is probably something you should ask Devart. Helen
