Re: Batch insert in CTAS/MatView code

On 06/03/2019 22:06, Paul Guo wrote:
> The patch also modifies heap_multi_insert() a bit to do a bit further 
> code-level optimization by using static memory, instead of using memory 
> context and dynamic allocation.

If toasting is required, heap_prepare_insert() creates a palloc'd tuple. 
That is still leaked to the current memory context.

Leaking into the current memory context is not a bad thing, because 
resetting a memory context is faster than doing a lot of pfree() calls. 
The callers just need to be prepared for that, and use a short-lived 
memory context.

> By the way, while looking at the code, I noticed that there are 9 local 
> arrays with large length in toast_insert_or_update() which seems to be a 
> risk of stack overflow. Maybe we should put it as static or global.

Hmm. We currently reserve 512 kB between the kernel's limit, and the 
limit we check in check_stack_depth(). See STACK_DEPTH_SLOP. Those 
arrays add up to 52800 bytes on a 64-bit maching, if I did my math 
right. So there's still a lot of headroom. I agree that it nevertheless 
seems a bit excessive, though.

> With the patch,
> 
> Time: 4728.142 ms (00:04.728)
> Time: 14203.983 ms (00:14.204)
> Time: 1008.669 ms (00:01.009)
> 
> Baseline,
> Time: 11096.146 ms (00:11.096)
> Time: 13106.741 ms (00:13.107)
> Time: 1100.174 ms (00:01.100)

Nice speedup!

> While for toast and large column size there is < 10% decrease but for 
> small column size the improvement is super good. Actually if I hardcode 
> the batch count as 4 all test cases are better but the improvement for 
> small column size is smaller than that with current patch. Pretty much 
> the number 4 is quite case specific so I can not hardcode that in the 
> patch. Of course we could further tune that but the current value seems 
> to be a good trade-off?

Have you done any profiling, on why the multi-insert is slower with 
large tuples? In principle, I don't see why it should be slower.

- Heikki