Re: [sqlite] WAL and pragma uncommitted
On 02/02/2018 10:00 PM, Hannah Massey wrote: Does anybody know why I would be getting SQLITE_BUSY when closing the database connection in each thread even though I have finalized all prepared statements and am not doing any outstanding commands on the database? I have tests that need to copy the database file once the application has finished and then delete it to clear up and they cannot delete the file because "it is in use by another process". Nothing else is accessing this database. Is there something else that must be done when working in WAL mode or when accessing from multiple threads as it has only started having this problem since I changed to having multiple reader threads and one write thread. Nothing else to do. If sqlite3_close() returns SQLITE_BUSY, then SQLite thinks there are one or more unfinalized statements or ongoing backup operations (open sqlite3_backup* handles). For debugging purposes, use sqlite3_next_stmt() to find the unfinalized statements after close() returns SQLITE_BUSY, and perhaps sqlite3_sql() to identify where in the app they come from. http://sqlite.org/c3ref/next_stmt.html http://sqlite.org/c3ref/expanded_sql.html Dan. Many Thanks in advance On 22 January 2018 at 09:37, Hannah Masseywrote: ok thanks. So looks like I'm going to try WAL mode with one connection to the database per thread and accessing the database using SQLITE_OPEN_NOMUTEX., no shared-cache mode, no pragma read_uncommitted. Thanks for the advice. On 20 January 2018 at 19:49, Dan Kennedy wrote: On 01/19/2018 11:26 PM, Hannah Massey wrote: Currently we access a single SQLite database in a single thread but I am working on changing this as performance has become a real problem. We will be using WAL mode and there will be one thread for writes and multiple threads for reads. For many cases, speed will be of a priority and it will not matter if the data returned from a read is slightly out of date so I can considering using #pragma uncommitted in some of the reader threads. Will #pragma uncommitted work in WAL mode and will it have the effect I'm looking for (where the read will be faster because it can ignore the recently written information in the WAL File) and simply use the database file only? Don't use "PRAGMA read_uncommitted". It is a no-op unless you turn on shared-cache mode. And using shared-cache mode reduces the concurrency provided by using wal mode. Dan. ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
Does anybody know why I would be getting SQLITE_BUSY when closing the database connection in each thread even though I have finalized all prepared statements and am not doing any outstanding commands on the database? I have tests that need to copy the database file once the application has finished and then delete it to clear up and they cannot delete the file because "it is in use by another process". Nothing else is accessing this database. Is there something else that must be done when working in WAL mode or when accessing from multiple threads as it has only started having this problem since I changed to having multiple reader threads and one write thread. Many Thanks in advance On 22 January 2018 at 09:37, Hannah Masseywrote: > ok thanks. So looks like I'm going to try WAL mode with one connection to > the database per thread and accessing the database using > SQLITE_OPEN_NOMUTEX., no shared-cache mode, no pragma read_uncommitted. > Thanks for the advice. > > On 20 January 2018 at 19:49, Dan Kennedy wrote: > >> On 01/19/2018 11:26 PM, Hannah Massey wrote: >> >>> Currently we access a single SQLite database in a single thread but I am >>> working on changing this as performance has become a real problem. We >>> will >>> be using WAL mode and there will be one thread for writes and multiple >>> threads for reads. For many cases, speed will be of a priority and it >>> will >>> not matter if the data returned from a read is slightly out of date so I >>> can considering using #pragma uncommitted in some of the reader threads. >>> Will #pragma uncommitted work in WAL mode and will it have the effect I'm >>> looking for (where the read will be faster because it can ignore the >>> recently written information in the WAL File) and simply use the database >>> file only? >>> >> >> Don't use "PRAGMA read_uncommitted". It is a no-op unless you turn on >> shared-cache mode. And using shared-cache mode reduces the concurrency >> provided by using wal mode. >> >> Dan. >> >> >> >> ___ >> sqlite-users mailing list >> sqlite-users@mailinglists.sqlite.org >> http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users >> > > ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
ok thanks. So looks like I'm going to try WAL mode with one connection to the database per thread and accessing the database using SQLITE_OPEN_NOMUTEX., no shared-cache mode, no pragma read_uncommitted. Thanks for the advice. On 20 January 2018 at 19:49, Dan Kennedywrote: > On 01/19/2018 11:26 PM, Hannah Massey wrote: > >> Currently we access a single SQLite database in a single thread but I am >> working on changing this as performance has become a real problem. We will >> be using WAL mode and there will be one thread for writes and multiple >> threads for reads. For many cases, speed will be of a priority and it will >> not matter if the data returned from a read is slightly out of date so I >> can considering using #pragma uncommitted in some of the reader threads. >> Will #pragma uncommitted work in WAL mode and will it have the effect I'm >> looking for (where the read will be faster because it can ignore the >> recently written information in the WAL File) and simply use the database >> file only? >> > > Don't use "PRAGMA read_uncommitted". It is a no-op unless you turn on > shared-cache mode. And using shared-cache mode reduces the concurrency > provided by using wal mode. > > Dan. > > > > ___ > sqlite-users mailing list > sqlite-users@mailinglists.sqlite.org > http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users > ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
On 01/19/2018 11:26 PM, Hannah Massey wrote: Currently we access a single SQLite database in a single thread but I am working on changing this as performance has become a real problem. We will be using WAL mode and there will be one thread for writes and multiple threads for reads. For many cases, speed will be of a priority and it will not matter if the data returned from a read is slightly out of date so I can considering using #pragma uncommitted in some of the reader threads. Will #pragma uncommitted work in WAL mode and will it have the effect I'm looking for (where the read will be faster because it can ignore the recently written information in the WAL File) and simply use the database file only? Don't use "PRAGMA read_uncommitted". It is a no-op unless you turn on shared-cache mode. And using shared-cache mode reduces the concurrency provided by using wal mode. Dan. ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
On 19 Jan 2018, at 4:46pm, Deon Brewiswrote: > If you start with: > INSERT INTO Woz(Foo, Bar) Values(1,1) > > And a (normal) writer thread updates the 2 columns: > UPDATE Woz SET Foo=2, Bar=2 > > Can a read_uncommitted thread read the value from the row as: > Foo=1, Bar=2 > ? No. The very least you can get is statement-level consistency. If that PRAGMA is on you might get (!,1) or (2,2), you just can’t predict which you’ll get. Simon. ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
Deon Brewis wrote: > What is the level of consistency (or rather inconsistency) for > read_uncommitted? In read_uncommited mode, read-only transactions to not take the database file lock. However, most sqlite3_xxx() function calls still lock the in-memory database object(s) (this is required for any multi-threaded accesses), so other threads can see only the complete changes made while a lock was held. In particular, sqlite3_step() takes a single lock around all VDBE statements it executes, so what you end up with is statement-level atomicy. Regards, Clemens ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
Hannah Massey wrote: > If I use separate connections for the reading threads then is there an > advantage to using "shared cache" for those connections? The shared cache would be useful to reduce memory usage (which should not be a concern except in embedded systems), but concurrent accesses to the same data structure need locking. This locking is done inside most sqlite3_xxx() function calls (see sqlite3_db_mutex()), and independent from the transaction locking. So for maximum performance, use one connection per thread without shared-cache mode. This allows you to run all reading threads in parallel, even when they are accessing the same database, and to use SQLITE_OPEN_NOMUTEX to avoid the locking overhead. (If the lock is not actually contested, the locking overhead would probably be too small to notice.) Regards, Clemens ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
What is the level of consistency (or rather inconsistency) for read_uncommitted? If you start with: INSERT INTO Woz(Foo, Bar) Values(1,1) And a (normal) writer thread updates the 2 columns: UPDATE Woz SET Foo=2, Bar=2 Can a read_uncommitted thread read the value from the row as: Foo=1, Bar=2 ? And if so, what about something like: UPDATE Woz SET Foo=2, Bar=x'12345789' Can a read_uncommitted thread read: Foo=x'1234' i.e. A partially updated column? I would assume that for a transactional update across 2 rows all bets will be off. What if a row was deleted, can a read_uncommitted read cause a crash? - Deon -Original Message- From: sqlite-users [mailto:sqlite-users-boun...@mailinglists.sqlite.org] On Behalf Of Simon Slavin Sent: Friday, January 19, 2018 8:36 AM To: SQLite mailing list <sqlite-users@mailinglists.sqlite.org> Subject: Re: [sqlite] WAL and pragma uncommitted On 19 Jan 2018, at 4:26pm, Hannah Massey <hannah.bea...@gmail.com> wrote: > Will #pragma uncommitted work in WAL mode and will it have the effect > I'm looking for (where the read will be faster because it can ignore > the recently written information in the WAL File) and simply use the > database file only? The command you’re talking about is PRAGMA read_uncommitted = boolean You may find you don’t need it. Switch to WAL mode and try it without that PRAGMA. It works more or less the way you described: one connection can write while other read, and nothing will block anything else. Pay a lot of attention to how many connections you’re using. You might want one connection for the writing, and another for all the reading threads. Or you might want separate connections for the reading threads too. Testing various approaches will let you find the solution which best suits your programming and timing requirements. Simon. ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org https://nam01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmailinglists.sqlite.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fsqlite-users=02%7C01%7C%7C7304a7658ee04115fe6508d55f5abc8e%7C84df9e7fe9f640afb435%7C1%7C0%7C636519765662668955=o6sR0H3YrZMdFUm9OwJ3CjyYi3J9s0MapK2%2BBeoiBuY%3D=0 ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
ok thanks for your advice. If I use separate connections for the reading threads then is there an advantage to using "shared cache" for those connections? On 19 January 2018 at 16:35, Simon Slavinwrote: > On 19 Jan 2018, at 4:26pm, Hannah Massey wrote: > > > Will #pragma uncommitted work in WAL mode and will it have the effect I'm > > looking for (where the read will be faster because it can ignore the > > recently written information in the WAL File) and simply use the database > > file only? > > The command you’re talking about is > > PRAGMA read_uncommitted = boolean > > You may find you don’t need it. Switch to WAL mode and try it without > that PRAGMA. It works more or less the way you described: one connection > can write while other read, and nothing will block anything else. > > Pay a lot of attention to how many connections you’re using. You might > want one connection for the writing, and another for all the reading > threads. Or you might want separate connections for the reading threads > too. Testing various approaches will let you find the solution which best > suits your programming and timing requirements. > > Simon. > ___ > sqlite-users mailing list > sqlite-users@mailinglists.sqlite.org > http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users > ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] WAL and pragma uncommitted
On 19 Jan 2018, at 4:26pm, Hannah Masseywrote: > Will #pragma uncommitted work in WAL mode and will it have the effect I'm > looking for (where the read will be faster because it can ignore the > recently written information in the WAL File) and simply use the database > file only? The command you’re talking about is PRAGMA read_uncommitted = boolean You may find you don’t need it. Switch to WAL mode and try it without that PRAGMA. It works more or less the way you described: one connection can write while other read, and nothing will block anything else. Pay a lot of attention to how many connections you’re using. You might want one connection for the writing, and another for all the reading threads. Or you might want separate connections for the reading threads too. Testing various approaches will let you find the solution which best suits your programming and timing requirements. Simon. ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
[sqlite] WAL and pragma uncommitted
Currently we access a single SQLite database in a single thread but I am working on changing this as performance has become a real problem. We will be using WAL mode and there will be one thread for writes and multiple threads for reads. For many cases, speed will be of a priority and it will not matter if the data returned from a read is slightly out of date so I can considering using #pragma uncommitted in some of the reader threads. Will #pragma uncommitted work in WAL mode and will it have the effect I'm looking for (where the read will be faster because it can ignore the recently written information in the WAL File) and simply use the database file only? Thanks ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users