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Hopefully better sort.c optimization comments.

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Related to #2346.
This commit is contained in:
antirez 2015-02-03 15:38:21 +01:00
parent 5fbb36f9e7
commit 96abf65900
1 changed files with 9 additions and 15 deletions
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24
src/sort.c
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@ -322,16 +322,16 @@ void sortCommand(redisClient *c) {
} }
if (end >= vectorlen) end = vectorlen-1; if (end >= vectorlen) end = vectorlen-1;
/* Optimization: /* Whenever possible, we load elements into the output array in a more
* direct way. This is possible if:
* *
* 1) if the object to sort is a sorted set. * 1) The object to sort is a sorted set or a list (internally sorted).
* 2) There is nothing to sort as dontsort is true (BY <constant string>). * 2) There is nothing to sort as dontsort is true (BY <constant string>).
* 3) We have a LIMIT option that actually reduces the number of elements
* to fetch.
* *
* In this case to load all the objects in the vector is a huge waste of * In this special case, if we have a LIMIT option that actually reduces
* resources. We just allocate a vector that is big enough for the selected * the number of elements to fetch, we also optimize to just load the
* range length, and make sure to load just this part in the vector. */ * range we are interested in and allocating a vector that is big enough
* for the selected range length. */
if ((sortval->type == REDIS_ZSET || sortval->type == REDIS_LIST) && if ((sortval->type == REDIS_ZSET || sortval->type == REDIS_LIST) &&
dontsort && dontsort &&
(start != 0 || end != vectorlen-1)) (start != 0 || end != vectorlen-1))
@ -364,10 +364,7 @@ void sortCommand(redisClient *c) {
j++; j++;
} }
listTypeReleaseIterator(li); listTypeReleaseIterator(li);
/* The code producing the output does not know that in the case of /* Fix start/end: output code is not aware of this optimization. */
* sorted set, 'dontsort', and LIMIT, we are able to get just the
* range, already sorted, so we need to adjust "start" and "end"
* to make sure start is set to 0. */
end -= start; end -= start;
start = 0; start = 0;
} }
@ -427,10 +424,7 @@ void sortCommand(redisClient *c) {
j++; j++;
ln = desc ? ln->backward : ln->level[0].forward; ln = desc ? ln->backward : ln->level[0].forward;
} }
/* The code producing the output does not know that in the case of /* Fix start/end: output code is not aware of this optimization. */
* sorted set, 'dontsort', and LIMIT, we are able to get just the
* range, already sorted, so we need to adjust "start" and "end"
* to make sure start is set to 0. */
end -= start; end -= start;
start = 0; start = 0;
} else if (sortval->type == REDIS_ZSET) { } else if (sortval->type == REDIS_ZSET) {