Re: [HACKERS] path toward faster partition pruning

On 10 November 2017 at 16:30, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> In 0002, bms_add_range has a bit naive-looking loop
>
> +       while (wordnum <= uwordnum)
> +       {
> +               bitmapword mask = (bitmapword) ~0;
> +
> +               /* If working on the lower word, zero out bits below 'lower'. */
> +               if (wordnum == lwordnum)
> +               {
> +                       int lbitnum = BITNUM(lower);
> +                       mask >>= lbitnum;
> +                       mask <<= lbitnum;
> +               }
> +
> +               /* Likewise, if working on the upper word, zero bits above 'upper' */
> +               if (wordnum == uwordnum)
> +               {
> +                       int ushiftbits = BITS_PER_BITMAPWORD - (BITNUM(upper) + 1);
> +                       mask <<= ushiftbits;
> +                       mask >>= ushiftbits;
> +               }
> +
> +               a->words[wordnum++] |= mask;
> +       }
>
> Without some aggressive optimization, the loop takes most of the
> time to check-and-jump for nothing especially with many
> partitions and somewhat unintuitive.
>
> The following uses a bit tricky bitmap operation but
> is straightforward as a whole.
>
> =====
> /* fill the bits upper from BITNUM(lower) (0-based) of the first word */
> a->workds[wordnum++] += ~(bitmapword)((1 << BITNUM(lower)) - 1);
>
> /* fill up intermediate words */
> while (wordnum < uwordnum)
>    a->words[wordnum++] = ~(bitmapword) 0;
>
> /* fill up to BITNUM(upper) bit (0-based) of the last word */
> a->workds[wordnum++] |=
>      (~(bitmapword) 0) >> (BITS_PER_BITMAPWORD - (BITNUM(upper) - 1));
> =====

No objections here for making bms_add_range() perform better, but this
is not going to work when lwordnum == uwordnum. You'd need to special
case that. I didn't think it was worth the trouble, but maybe it is...

I assume the += should be |=.

-- David Rowley                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services


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