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De-obfuscate the comment in tsrank.c's calc_rank_or().

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Oleg's original comment was intelligible only to him.
Aleksander has reverse-engineered what seems like a plausible
explanation of what the code is trying to do, so replace the
comment with that.  (Also, re-order the final expression to
match the new comment.)

In passing, this makes the comment satisfy our usual formatting
conventions.  pgindent has let it pass as-is so far, but planned
changes would mess it up without some sort of intervention.

Author: Aleksander Alekseev <aleksander@tigerdata.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAJ7c6TO0xvunpeOv89i1eKQBhKF9=GEETkTz+yAGs1xGYH25MQ@mail.gmail.com
This commit is contained in:
Tom Lane 2026-05-12 15:04:42 -04:00
parent 06fccab4c6
commit 163f20ca12
1 changed files with 31 additions and 8 deletions
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39
src/backend/utils/adt/tsrank.c
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@ -313,6 +313,7 @@ calc_rank_or(const float *w, TSVector t, TSQuery q)
while (entry - firstentry < nitem)
{
/* Identify the weight data to use */
if (entry->haspos)
{
dimt = POSDATALEN(t, entry);
@ -324,6 +325,33 @@ calc_rank_or(const float *w, TSVector t, TSQuery q)
post = posnull.pos;
}
/*
* Compute the score for this term, then add it to "res".
*
* The ideal score for a term is the weighted harmonic sum:
*
* resj = sum(wi / i^2, i = 1..noccurrences)
*
* where wi is the weight of the i-th occurrence and weights are
* sorted in descending order so that the highest-weight
* occurrence gets the smallest divisor (i=1) and thus contributes
* the most.
*
* This result should be divided by pi^2/6 ~= 1.64493406685, which
* is the limit of sum(1/i^2, i=1..inf). This normalizes the score
* to the [0, 1] range.
*
* As an approximation for efficiency, we skip doing a sort and
* instead just promote the single highest-weight occurrence to
* position i=1. This is done by taking the raw (unsorted) sum
* resj, subtracting the maximum weight's actual contribution
* wjm/(jm+1)^2, and adding back its corrected contribution
* wjm/1^2 = wjm.
*
* The remaining occurrences are left in their original order.
*/
/* calculate harmonic sum without re-ordering */
resj = 0.0;
wjm = -1.0;
jm = 0;
@ -336,14 +364,9 @@ calc_rank_or(const float *w, TSVector t, TSQuery q)
jm = j;
}
}
/*
limit (sum(1/i^2),i=1,inf) = pi^2/6
resj = sum(wi/i^2),i=1,noccurrence,
wi - should be sorted desc,
don't sort for now, just choose maximum weight. This should be corrected
Oleg Bartunov
*/
res = res + (wjm + resj - wjm / ((jm + 1) * (jm + 1))) / 1.64493406685;
/* apply correction as explained above, then add to res */
res = res + (resj - wjm / ((jm + 1) * (jm + 1)) + wjm) / 1.64493406685;
entry++;
}