Re: Performance optimization of btree binary search

On 2013-12-04 18:48:44 -0500, Robert Haas wrote:
>  * When a type narrower than Datum is stored in a Datum, we place it in the
>  * low-order bits and are careful that the DatumGetXXX macro for it discards
>  * the unused high-order bits (as opposed to, say, assuming they are zero).
>  * This is needed to support old-style user-defined functions, since depending
>  * on architecture and compiler, the return value of a function returning char
>  * or short may contain garbage when called as if it returned Datum.
> 
> And record_image_eq does a rather elaborate dance around here, calling
> the appropriate GET_x_BYTES macro depending on the type-width.  If we
> can really count on the high-order bits to be zero, that's all
> completely unnecessary tomfoolery.

I don't think we can get rid of that dance in record_image_eq - it very
well could used on records originally generated when those bits haven't
been guaranteed to be zeroed.
Other usecases where the appropriate DatumGetFoo() macros are used don't
have a problem with that since it's cleared again there, but in
record_image_eq we can't rely on that.

Or am I missing something?

Greetings,

Andres Freund

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