Re: Small patch to fix an O(N^2) problem in foreign keys

On 09/08/2015 08:49 AM, Kevin Grittner wrote:
> Jan Wieck <jan@wi3ck.info> wrote:
>
>> The problem is a cache introduced in commit 45ba4247 that improves
>
> That's a bit off; 45ba424f seems to be what you mean.

Copy paste over paper.


>
>> foreign key lookups during bulk updates when the FK value does not
>> change. When restoring a schema dump from a database with many (say
>> 100,000) foreign keys, this cache is growing very big and every ALTER
>> TABLE command is causing a InvalidateConstraintCacheCallBack(), which
>> does a sequential hash table scan.
>>
>> The patch uses a heuristic method of detecting when the hash table
>> should be destroyed and recreated. InvalidateConstraintCacheCallBack()
>> adds the current size of the hash table to a counter. When that sum
>> reaches 1,000,000, the hash table is flushed. This improves the schema
>> restore of a database with 100,000 foreign keys by factor 3.
>>
>> According to my tests the patch does not interfere with the bulk
>> updates, the original feature was supposed to improve.
>
> In the real-world customer case that caused you to look into this,
> I thought 45ba424f drove schema-only restore time from 2 hours to
> about 25 hours, and that this patch takes it back down to 2 hours.
> Am I remembering right?  And this came about because it added
> 20-some hours to a pg_upgrade run?
From 2 hours to >50, but yes, this is that case.

>
> If there are no objections, I will push this as a bug fix back to
> 9.3, where the performance regression was introduced.


Regards, Jan


-- 
Jan Wieck
Senior Software Engineer
http://slony.info