Thanks for your insight! But I'm still worried about the performance of
IN ( big list of values ). Can you tell me how it is implemented?
Suppose I have SELECT a FROM b WHERE c IN (1, 4, 5, 117, 118, 119, ...,
1) If I put 200 values there, does it do 200 individual SELECTs
internally, and union them? Or does it notice that c has a "UNIQUE"
index and thus at most one row can be returned per SELECT, and does them
all at once?
2) If I want to get just the primary key, or join with another table
based on just the primary key, does this query ever touch the disk
(assuming the index is in memory, which I think it always is -- correct
me if I'm wrong about that).
The way I would recommend doing it (for BTREE indexes, anyway) is to
sort the values in ascending order, and do the search in one pass
through the index. The index is already in memory, and it would be
straightforward to modify a binary search algorithm to find the rows
corresponding to monotonically ascending values of the primary key, all
in one pass.
Even if the binary search algorithm is run 200 or 2000 times for a list,
it would still be faster than hitting the disk. (Even though the CPU
cache performance would be worse.)
Can you let me know the specifics of it, and especially how I can avoid
hitting the I/O bottlenecks?
On 3/29/11 4:17 PM, Peter Brawley wrote:
> > Why not optimize the IN ( ... ) to do the same type of thing?
> If the argument to IN() is a list of values, it'll be OK. If it's a
> SELECT, in 5.0 it will be slower than molasses (see "The unbearable
> slowness of IN()" at http://www.artfulsoftware.com/queries.php.
> > I always tried to avoid joins because I am planning to horizontally
> partition my data.
> A severe & unfortunate constraint. Can't help you there.
> On 3/29/2011 1:27 PM, Gregory Magarshak wrote:
>> Yes, this would be fine. But often, the list of friends is obtained
>> from a social network like facebook, and is not stored internally.
>> Basically, I obtain the friend list in a request to facebook, and
>> then see which of those users have created things. So would I have to
>> create a temporary table and insert all those uids just to make a
>> join? Why not optimize the IN ( ... ) to do the same type of thing?
>> There is also a second problem: I want to use MySQL Cluster, because
>> I expect to have many users. Would it be efficient to use JOIN
>> between the friends table and the articles table? Both tables are
>> partitioned by user_id as the primary key, so the join would have to
>> hit many different nodes. I always tried to avoid joins because I am
>> planning to horizontally partition my data. But if MySQL cluster can
>> handle this join transparently and split it up based on the
>> partition, then that's fine. Do you have any info on this?
>> On 3/29/11 2:10 PM, Peter Brawley wrote:
>>> > How can I quickly find all the articles written by this user's
>>> friends, and not just random articles?
>>> Taking the simplest possible case, with table
>>> friends(userID,friendID) where each friendID refers to a userID in
>>> another row, the friends of userID u are ...
>>> select friendID from user where userID=u;
>>> so articles by those friends of u are ...
>>> select a.* from article a join ( select friendID from user where
>>> userID=u ) f on a.userID=f.friendID;
>>> On 3/29/2011 12:50 PM, Gregory Magarshak wrote:
>>>> Hey there. My company writes a lot of social applications, and
>>>> there is one operation that is very common, but I don't know if
>>>> MySQL supports it in a good way. I thought I'd write to this list
>>>> for two reasons:
>>>> 1) Maybe MySQL has a good way to do this, and I just don't know
>>>> about it
>>>> 2) Propose to MySQL developers a simple algorithm which would
>>>> greatly improve MySQL support for social networking apps.
>>>> Here is the situation. Let's say I have built a social
>>>> networking application where people create and edit some item
>>>> (article, photo, music mix, whatever). Now, a typical user logs in,
>>>> and this user has 3000 friends. How can I quickly find all the
>>>> articles written by this user's friends, and not just random articles?
>>>> Ideally, I would want to write something like this:
>>>> SELECT * FROM article WHERE user_id IN (345789, 324875, 398,
>>>> ..., 349580)
>>>> basically, execute a query with a huge IN ( ... ). Maybe if
>>>> this would exceed the buffer size for the MySQL wire protocol, I
>>>> would break up the list into several lists, and execute several
>>>> queries, and union the results together myself.
>>>> But my point is, this is very common for social networking
>>>> apps. Every app wants to show "the X created by your friends", or
>>>> "friends of yours (given some list from a social network) who have
>>>> taken action X".
>>>> Here is how I would do it if I had raw access to the MySQL
>>>> index in memory:
>>>> a) Sort the list of entries in the IN, in ascending order.
>>>> b) Do *ONE* binary search through the index (assuming it's a
>>>> BTREE index) and get them all in one pass. If it's a HASH index or
>>>> something, I would have to look up each one individually.
>>>> The benefits of this approach would be that this common
>>>> operation would be done extremely quickly. If the index fits
>>>> entirely in memory, and I just want to get the primary keys (i.e.
>>>> get the list of friends who did X), the disk isn't even touched. In
>>>> addition, for BTREE indexes, I would just need ONE binary search,
>>>> because the entries have been sorted in ascending order.
>>>> Does MySQL have something like this? And if not, perhaps you
>>>> can add it in the next version? It would really boost MySQL's
>>>> support for social networking apps tremendously. Alternative, how
>>>> can I add this to my MySQL? Any advice would be appreciated.
>>>> Gregory Magarshak