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Improve sift up/down code in binaryheap.c and logtape.c.

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Borrow the logic that's long been used in tuplesort.c: instead
of physically swapping the data in two heap entries, keep the
value that's being sifted up or down in a local variable, and
just move the other values as necessary.  This makes the code
shorter as well as faster.  It's not clear that any current
callers are really time-critical enough to notice, but we
might as well code heap maintenance the same way everywhere.

Ma Liangzhu and Tom Lane

Discussion: https://postgr.es/m/17336-fc4e522d26a750fd@postgresql.org
This commit is contained in:
Tom Lane
2021-12-14 13:35:22 -05:00
parent 2de3c1015c
commit a2ff18e89f
2 changed files with 63 additions and 61 deletions
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70
src/backend/lib/binaryheap.c
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@@ -19,7 +19,6 @@
static void sift_down(binaryheap *heap, int node_off);
static void sift_up(binaryheap *heap, int node_off);
static inline void swap_nodes(binaryheap *heap, int a, int b);
/*
* binaryheap_allocate
@@ -173,24 +172,28 @@ binaryheap_first(binaryheap *heap)
Datum
binaryheap_remove_first(binaryheap *heap)
{
Datum result;
Assert(!binaryheap_empty(heap) && heap->bh_has_heap_property);
/* extract the root node, which will be the result */
result = heap->bh_nodes[0];
/* easy if heap contains one element */
if (heap->bh_size == 1)
{
heap->bh_size--;
return heap->bh_nodes[0];
return result;
}
/*
* Swap the root and last nodes, decrease the size of the heap (i.e.
* remove the former root node) and sift the new root node down to its
* correct position.
* Remove the last node, placing it in the vacated root entry, and sift
* the new root node down to its correct position.
*/
swap_nodes(heap, 0, heap->bh_size - 1);
heap->bh_size--;
heap->bh_nodes[0] = heap->bh_nodes[--heap->bh_size];
sift_down(heap, 0);
return heap->bh_nodes[heap->bh_size];
return result;
}
/*
@@ -211,19 +214,6 @@ binaryheap_replace_first(binaryheap *heap, Datum d)
sift_down(heap, 0);
}
/*
* Swap the contents of two nodes.
*/
static inline void
swap_nodes(binaryheap *heap, int a, int b)
{
Datum swap;
swap = heap->bh_nodes[a];
heap->bh_nodes[a] = heap->bh_nodes[b];
heap->bh_nodes[b] = swap;
}
/*
* Sift a node up to the highest position it can hold according to the
* comparator.
@@ -231,29 +221,40 @@ swap_nodes(binaryheap *heap, int a, int b)
static void
sift_up(binaryheap *heap, int node_off)
{
Datum node_val = heap->bh_nodes[node_off];
/*
* Within the loop, the node_off'th array entry is a "hole" that
* notionally holds node_val, but we don't actually store node_val there
* till the end, saving some unnecessary data copying steps.
*/
while (node_off != 0)
{
int cmp;
int parent_off;
Datum parent_val;
/*
* If this node is smaller than its parent, the heap condition is
* satisfied, and we're done.
*/
parent_off = parent_offset(node_off);
cmp = heap->bh_compare(heap->bh_nodes[node_off],
heap->bh_nodes[parent_off],
parent_val = heap->bh_nodes[parent_off];
cmp = heap->bh_compare(node_val,
parent_val,
heap->bh_arg);
if (cmp <= 0)
break;
/*
* Otherwise, swap the node and its parent and go on to check the
* node's new parent.
* Otherwise, swap the parent value with the hole, and go on to check
* the node's new parent.
*/
swap_nodes(heap, node_off, parent_off);
heap->bh_nodes[node_off] = parent_val;
node_off = parent_off;
}
/* Re-fill the hole */
heap->bh_nodes[node_off] = node_val;
}
/*
@@ -263,6 +264,13 @@ sift_up(binaryheap *heap, int node_off)
static void
sift_down(binaryheap *heap, int node_off)
{
Datum node_val = heap->bh_nodes[node_off];
/*
* Within the loop, the node_off'th array entry is a "hole" that
* notionally holds node_val, but we don't actually store node_val there
* till the end, saving some unnecessary data copying steps.
*/
while (true)
{
int left_off = left_offset(node_off);
@@ -271,14 +279,14 @@ sift_down(binaryheap *heap, int node_off)
/* Is the left child larger than the parent? */
if (left_off < heap->bh_size &&
heap->bh_compare(heap->bh_nodes[node_off],
heap->bh_compare(node_val,
heap->bh_nodes[left_off],
heap->bh_arg) < 0)
swap_off = left_off;
/* Is the right child larger than the parent? */
if (right_off < heap->bh_size &&
heap->bh_compare(heap->bh_nodes[node_off],
heap->bh_compare(node_val,
heap->bh_nodes[right_off],
heap->bh_arg) < 0)
{
@@ -298,10 +306,12 @@ sift_down(binaryheap *heap, int node_off)
break;
/*
* Otherwise, swap the node with the child that violates the heap
* Otherwise, swap the hole with the child that violates the heap
* property; then go on to check its children.
*/
swap_nodes(heap, swap_off, node_off);
heap->bh_nodes[node_off] = heap->bh_nodes[swap_off];
node_off = swap_off;
}
/* Re-fill the hole */
heap->bh_nodes[node_off] = node_val;
}