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Add bms_next_member(), and use it where appropriate.

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This patch adds a way of iterating through the members of a bitmapset
nondestructively, unlike the old way with bms_first_member().  While
bms_next_member() is very slightly slower than bms_first_member()
(at least for typical-size bitmapsets), eliminating the need to palloc
and pfree a temporary copy of the target bitmapset is a significant win.
So this method should be preferred in all cases where a temporary copy
would be necessary.

Tom Lane, with suggestions from Dean Rasheed and David Rowley
This commit is contained in:
Tom Lane
2014-11-28 13:37:25 -05:00
parent 96d66bcfc6
commit f4e031c662
13 changed files with 114 additions and 76 deletions
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68
src/backend/nodes/bitmapset.c
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@ -793,7 +793,7 @@ bms_join(Bitmapset *a, Bitmapset *b)
return result;
}
/*----------
/*
* bms_first_member - find and remove first member of a set
*
* Returns -1 if set is empty. NB: set is destructively modified!
@ -801,11 +801,11 @@ bms_join(Bitmapset *a, Bitmapset *b)
* This is intended as support for iterating through the members of a set.
* The typical pattern is
*
* tmpset = bms_copy(inputset);
* while ((x = bms_first_member(tmpset)) >= 0)
* while ((x = bms_first_member(inputset)) >= 0)
* process member x;
* bms_free(tmpset);
*----------
*
* CAUTION: this destroys the content of "inputset". If the set must
* not be modified, use bms_next_member instead.
*/
int
bms_first_member(Bitmapset *a)
@ -840,6 +840,64 @@ bms_first_member(Bitmapset *a)
return -1;
}
/*
* bms_next_member - find next member of a set
*
* Returns smallest member greater than "prevbit", or -2 if there is none.
* "prevbit" must NOT be less than -1, or the behavior is unpredictable.
*
* This is intended as support for iterating through the members of a set.
* The typical pattern is
*
* x = -1;
* while ((x = bms_next_member(inputset, x)) >= 0)
* process member x;
*
* Notice that when there are no more members, we return -2, not -1 as you
* might expect. The rationale for that is to allow distinguishing the
* loop-not-started state (x == -1) from the loop-completed state (x == -2).
* It makes no difference in simple loop usage, but complex iteration logic
* might need such an ability.
*/
int
bms_next_member(const Bitmapset *a, int prevbit)
{
int nwords;
int wordnum;
bitmapword mask;
if (a == NULL)
return -2;
nwords = a->nwords;
prevbit++;
mask = (~(bitmapword) 0) << BITNUM(prevbit);
for (wordnum = WORDNUM(prevbit); wordnum < nwords; wordnum++)
{
bitmapword w = a->words[wordnum];
/* ignore bits before prevbit */
w &= mask;
if (w != 0)
{
int result;
result = wordnum * BITS_PER_BITMAPWORD;
while ((w & 255) == 0)
{
w >>= 8;
result += 8;
}
result += rightmost_one_pos[w & 255];
return result;
}
/* in subsequent words, consider all bits */
mask = (~(bitmapword) 0);
}
return -2;
}
/*
* bms_hash_value - compute a hash key for a Bitmapset
*