Fri, 19 Oct 2012 15:49:07 -0400 от "Igor Tandetnik" <itandet...@mvps.org>:
>Григорий Григоренко <grigore...@mail.ru> wrote:
>
> I am using prepared stmts to cache selected record for later use. I have a
> wrapper class, that has methods like AsString(name),
>
> AsFloat(name) etc to extract data for a column name.
>
>
>
> I prepare, bind and then do Sqlite3_step() to get record. Later I use
> sqlite3_column_XXX() to access data.
>
>
>
> My concern is - do open selective prepared stmts depend on something in db or
> lock something in db?
>
>
Yes, until you call sqlite3_reset or sqlite3_finalize. For as long as the
statement is "active" (meaning, sqlite3_reset or sqlite3_finalize has not been
called after the most recent sqlite3_step call), it holds a read transaction
open.
OK.
>
>
> Var Alice := Db.SelectRecord('SELECT * FROM client WHERE client_name = ?', [
> 'alice' ]);
>
>
>
> 1. If I drop record for "alice" from db and then access column data in
> prepared stmt will it work OK?
>
>
If the select statement wasn't reset or finalized, you won't be able to delete
a record from the database (or, in WAL journaling mode, you would be able to,
but the reader would still see original data).
What kind of error is returned in this case?
Actually, I should (and would) find out this via simple test app )
>
>
> 2. Can I drop client table while having such prepared stmt not closed?
>
>
What do you mean by "closed" here?
Finalized.
>
>
> 3. If I have a lot of such stmts (100 or more) will it somehow affect
> performance of SQLite or waste lots of memory?
>
>
No, not really. There is a reasonably small data structure associated with a
prepared statement, on the order of a few hundred bytes perhaps.
>
>
> Are there some kind of cursors for each selecting prepared stmt?
>
>
In a sense. After a call to sqlite3_step and before a call of sqlite3_reset or
sqlite3_finalize, you might think of a statement handle as a form of a cursor.
Each subsequent sqlite3_step call advances this cursor forward by one row.
>
>
> 4. If there is a cursor, maybe there is a way to disconnect a cursor from
> stmt keeping the single record still available?
>
>
No, short of making a copy of every column's value.
OK.
>
--
>
Igor Tandetnik
>
>
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