Re: [Tarantool-patches] [PATCH v4 10/16] sql: refactor AVG() function

[Mergen Imeev via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Thank you for the review! My answer below.

On Mon, Oct 04, 2021 at 11:53:19PM +0200, Vladislav Shpilevoy wrote:
> Thanks for the fixes!
> 
> > diff --git a/src/box/sql/func.c b/src/box/sql/func.c
> > index 8a9de1254..94ec811ef 100644
> > --- a/src/box/sql/func.c
> > +++ b/src/box/sql/func.c
> > @@ -102,6 +102,58 @@ fin_total(struct sql_context *ctx)
> 
> <...>
> 
> > +
> > +/** Finalizer for the AVG() function. */
> > +static void
> > +fin_avg(struct sql_context *ctx)
> > +{
> > +	assert(mem_is_null(ctx->pMem) || mem_is_bin(ctx->pMem));
> > +	if (mem_is_null(ctx->pMem))
> > +		return mem_set_null(ctx->pOut);
> > +	struct Mem *tmp = (struct Mem *)ctx->pMem->z;
> > +	uint32_t *count_val = (uint32_t *)(tmp + 1);
> > +	struct Mem sum;
> > +	mem_create(&sum);
> > +	mem_copy_as_ephemeral(&sum, tmp);
> > +	mem_destroy(tmp);
> > +	struct Mem count;
> > +	mem_create(&count);
> > +	mem_set_uint(&count, *count_val);
> > +	if (mem_div(&sum, &count, ctx->pOut) != 0)
> > +		ctx->is_aborted = true;
> 
> Consider this:
> 
> ====================
> @@ -141,17 +141,14 @@ fin_avg(struct sql_context *ctx)
>  	assert(mem_is_null(ctx->pMem) || mem_is_bin(ctx->pMem));
>  	if (mem_is_null(ctx->pMem))
>  		return mem_set_null(ctx->pOut);
> -	struct Mem *tmp = (struct Mem *)ctx->pMem->z;
> -	uint32_t *count_val = (uint32_t *)(tmp + 1);
> -	struct Mem sum;
> -	mem_create(&sum);
> -	mem_copy_as_ephemeral(&sum, tmp);
> -	mem_destroy(tmp);
> +	struct Mem *sum = (struct Mem *)ctx->pMem->z;
> +	uint32_t *count_val = (uint32_t *)(sum + 1);
>  	struct Mem count;
>  	mem_create(&count);
>  	mem_set_uint(&count, *count_val);
>  	if (mem_div(&sum, &count, ctx->pOut) != 0)
>  		ctx->is_aborted = true;
> +	mem_destroy(sum);
>  }
This will work, however, I think it will create some unnecessary restrictions
due to changes with pMem and pOut in a few patches. I suggest to apply part of
you diff with exception of mem_destroy(), which I sugget to replace by assert().
We have full control over this tmp/sum mem and we know, that there will be no
memory to free, so assert should be enough.

What do you think of this diff?

diff --git a/src/box/sql/func.c b/src/box/sql/func.c
index a811e55f9..8213704e9 100644
--- a/src/box/sql/func.c
+++ b/src/box/sql/func.c
@@ -141,16 +141,13 @@ fin_avg(struct sql_context *ctx)
 	assert(mem_is_null(ctx->pMem) || mem_is_bin(ctx->pMem));
 	if (mem_is_null(ctx->pMem))
 		return mem_set_null(ctx->pOut);
-	struct Mem *tmp = (struct Mem *)ctx->pMem->z;
-	uint32_t *count_val = (uint32_t *)(tmp + 1);
-	struct Mem sum;
-	mem_create(&sum);
-	mem_copy_as_ephemeral(&sum, tmp);
-	mem_destroy(tmp);
+	struct Mem *sum = (struct Mem *)ctx->pMem->z;
+	uint32_t *count_val = (uint32_t *)(sum + 1);
+	assert(!VdbeMemDynamic(sum) && sum->szMalloc == 0);
 	struct Mem count;
 	mem_create(&count);
 	mem_set_uint(&count, *count_val);
-	if (mem_div(&sum, &count, ctx->pOut) != 0)
+	if (mem_div(sum, &count, ctx->pOut) != 0)
 		ctx->is_aborted = true;
 }