Hi Ann, and the crew!
After pushing http://lists.mysql.com/commits/73707 for bug #43630
'A SELECT using ORDER BY and LIMIT sometimes returns no rows'
I have been looking at the remaining cases where we are returned too
few, or no, rows when doing a LIMIT query. It seems that when we find
a node
with a recordNumber >= 0 as the first node on a page, we forget to
expand
the key. I believed that this was deliberate, as the key should
already be
expanded, being identical to the last entry on the previous page.
Looking at this
in the debugger however, this does not seem to be the case.
When we start searching the first page that actually contains valid
entries,
IndexWalker::keyLength and IndexWalker::key has the following values:
(gdb) p keyLength
$4 = 2
(gdb) x/bt key
0x7fab4cfaf810: 10111111
(gdb) x/bt key+1
0x7fab4cfaf811: 11110000
whereas the index node we are looking at has the following:
(gdb) p node
$2 = {node = 0x7fab4cfa7750, nextNode = 0x7fab4cfa7755, offset = 0,
length = 1, key = 0x7fab4cfa7754 "\300\001", number = 329}
(gdb) p node.length
$3 = 1
(gdb) x/bt node.key
0x7fab4cfa7754: 11000000
The keys and keylengths do not match. The recordKey created from the
record when comparing in IndexWalker::getValidatedRecord, however, looks
the same as the node:
(gdb) p recordKey.keyLength
$5 = 1
(gdb) x/bt recordKey.key
0x7fab3fcb88e8: 11000000
Since all the fields have the same value, expanding the key for the
following
nodes is a no-op, because the offset will be 1 and the length 0.
Therefore we
miss a lot of rows until we reach a node with offset 0 and length 1.
This is
maybe a supernode, since it seems to be fully expanded?
If I add the following line of code:
int32 WalkIndex::getNextNode(void)
{
for (;; first = true)
{
if (first)
{
first = false;
recordNumber = node.getNumber();
if (recordNumber >= 0)
{
node.expandKey(&indexKey); // <==== This line
return recordNumber;
}
we only experience some cases where we miss 1 row, opposed to
often missing a lot. We still have the other problem with too many
rows though :)
Is it correct that we miss to expand the key here, or is it something
else that is fishy?
(I know there is fishy stuff here, but hopefully not regarding this
particular issue :) )
For the curious, a little information about the nodes searched is
presented below:
Page = page number
dup = number of entries reported as duplicates
nrec = number of entries that have no matching record
ncan = number of entries that had no visible records
nmis = number of entries who's key does not match with the key
generated from the record
nret = number of entries returned as valid entries
This is presented in search order
Page:137, dup:0, nrec:0, ncan:0, nmis:
35, nret:0
Page:138, dup:0, nrec:0, ncan:0, nmis:
1000, nret:0
Page:181, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:175, dup:0, nrec:0, ncan:0, nmis:
206, nret:465 (valid)
Page:136, dup:0, nrec:0, ncan:0, nmis:
515, nret:0
Page:219, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:147, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:174, dup:0, nrec:0, ncan:0, nmis:
2, nret:329 (valid)
Page:199, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:173, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:214, dup:1, nrec:0, ncan:0, nmis:
451, nret:0
Page:149, dup:1, nrec:0, ncan:0, nmis:
459, nret:0
Page:139, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:156, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:185, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:159, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:229, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:164, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:165, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:186, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:191, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:228, dup:0, nrec:0, ncan:0, nmis:
91, nret:0
Page:230, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:234, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:235, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:158, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:160, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:155, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:151, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:162, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:236, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:237, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:161, dup:0, nrec:0, ncan:0, nmis:
23, nret:0
Page:143, dup:1, nrec:0, ncan:0, nmis:
506, nret:0
Page:140, dup:0, nrec:0, ncan:0, nmis:
1, nret:0
Page:142, dup:0, nrec:0, ncan:0, nmis:
24, nret:0
Page:141, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Page:218, dup:0, nrec:0, ncan:0, nmis:
0, nret:0 (empty)
Number of empty pages: 11
Number of pages with vaild entries: 2
Number of pages processed: 38
This query returned 794 rows, while 1000 rows were expected
/Lars-Erik
