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Don't overwrite scan key in systable_beginscan()

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When systable_beginscan() and systable_beginscan_ordered() choose an
index scan, they remap the attribute numbers in the passed-in scan
keys to the attribute numbers of the index, and then write those
remapped attribute numbers back into the scan key passed by the
caller.  This second part is surprising and gratuitous.  It means that
a scan key cannot safely be used more than once (but it might
sometimes work, depending on circumstances).  Also, there is no value
in providing these remapped attribute numbers back to the caller,
since they can't do anything with that.

Fix that by making a copy of the scan keys passed by the caller and
make the modifications there.

Also, some code that had to work around the previous situation is
simplified.

Discussion: https://www.postgresql.org/message-id/flat/f8c739d9-f48d-4187-b214-df3391ba41ab@eisentraut.org
This commit is contained in:
Peter Eisentraut
2024-08-08 08:27:26 +02:00
parent 00c76cf21c
commit 811af9786b
3 changed files with 40 additions and 33 deletions
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24
src/backend/access/index/genam.c
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@@ -372,7 +372,7 @@ index_compute_xid_horizon_for_tuples(Relation irel,
* nkeys, key: scan keys
*
* The attribute numbers in the scan key should be set for the heap case.
* If we choose to index, we reset them to 1..n to reference the index
* If we choose to index, we convert them to 1..n to reference the index
* columns. Note this means there must be one scankey qualification per
* index column! This is checked by the Asserts in the normal, index-using
* case, but won't be checked if the heapscan path is taken.
@@ -420,17 +420,22 @@ systable_beginscan(Relation heapRelation,
if (irel)
{
int i;
ScanKey idxkey;
/* Change attribute numbers to be index column numbers. */
idxkey = palloc_array(ScanKeyData, nkeys);
/* Convert attribute numbers to be index column numbers. */
for (i = 0; i < nkeys; i++)
{
int j;
memcpy(&idxkey[i], &key[i], sizeof(ScanKeyData));
for (j = 0; j < IndexRelationGetNumberOfAttributes(irel); j++)
{
if (key[i].sk_attno == irel->rd_index->indkey.values[j])
{
key[i].sk_attno = j + 1;
idxkey[i].sk_attno = j + 1;
break;
}
}
@@ -440,7 +445,7 @@ systable_beginscan(Relation heapRelation,
sysscan->iscan = index_beginscan(heapRelation, irel,
snapshot, nkeys, 0);
index_rescan(sysscan->iscan, key, nkeys, NULL, 0);
index_rescan(sysscan->iscan, idxkey, nkeys, NULL, 0);
sysscan->scan = NULL;
}
else
@@ -648,6 +653,7 @@ systable_beginscan_ordered(Relation heapRelation,
{
SysScanDesc sysscan;
int i;
ScanKey idxkey;
/* REINDEX can probably be a hard error here ... */
if (ReindexIsProcessingIndex(RelationGetRelid(indexRelation)))
@@ -679,16 +685,20 @@ systable_beginscan_ordered(Relation heapRelation,
sysscan->snapshot = NULL;
}
/* Change attribute numbers to be index column numbers. */
idxkey = palloc_array(ScanKeyData, nkeys);
/* Convert attribute numbers to be index column numbers. */
for (i = 0; i < nkeys; i++)
{
int j;
memcpy(&idxkey[i], &key[i], sizeof(ScanKeyData));
for (j = 0; j < IndexRelationGetNumberOfAttributes(indexRelation); j++)
{
if (key[i].sk_attno == indexRelation->rd_index->indkey.values[j])
{
key[i].sk_attno = j + 1;
idxkey[i].sk_attno = j + 1;
break;
}
}
@@ -698,7 +708,7 @@ systable_beginscan_ordered(Relation heapRelation,
sysscan->iscan = index_beginscan(heapRelation, indexRelation,
snapshot, nkeys, 0);
index_rescan(sysscan->iscan, key, nkeys, NULL, 0);
index_rescan(sysscan->iscan, idxkey, nkeys, NULL, 0);
sysscan->scan = NULL;
return sysscan;