| From: | Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> |
|---|---|
| To: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
| Cc: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Mark Dilger <hornschnorter(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [HACKERS] PATCH: multivariate histograms and MCV lists |
| Date: | 2019-01-17 07:46:51 |
| Message-ID: | CAEZATCXDPLr44LR6t0rp4Ps0E-cwu6uijJ0dTr5H1aOft3COzw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, 17 Jan 2019 at 03:42, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
> 35. The evaluation order of this macro is wrong.
>
> #define ITEM_SIZE(ndims) \
> (ndims * (sizeof(uint16) + sizeof(bool)) + 2 * sizeof(double))
>
> You'd probably want ITEM_SIZE(10) to return 170, but:
>
> select (10 * (2 + 1) + 2 * 8);
> ?column?
> ----------
> 46
>
> Unsure why this does not cause a crash.
>
No, the code is actually correct, as explained in the comment above
it. Each item contains (ndims) copies of the uint16 index and the
boolean, but it always contains exactly 2 doubles, independent of
ndims.
> ndims should also have parenthesis around it in case someone does
> ITEM_SIZE(x + y), likewise for the other ITEM_* macros.
>
+1 on that point.
Regards,
Dean
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