Details
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Bug
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Status: Open
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Major
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Resolution: Unresolved
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Impala 2.12.0, Impala 2.13.0, Impala 3.1.0
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None
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None
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ghx-label-9
Description
With IMPALA-5547, we predict whether a join is an FK/PK join as follows.
// Iterate over all groups of conjuncts that belong to the same joined tuple id pair.
// For each group, we compute the join NDV of the rhs slots and compare it to the
// number of rows in the rhs table.
for (List<EqJoinConjunctScanSlots> fkPkCandidate: scanSlotsByJoinedTids.values()) {
double jointNdv = 1.0;
for (EqJoinConjunctScanSlots slots: fkPkCandidate) jointNdv *= slots.rhsNdv();
double rhsNumRows = fkPkCandidate.get(0).rhsNumRows();
if (jointNdv >= Math.round(rhsNumRows * (1.0 - FK_PK_MAX_STATS_DELTA_PERC))) {
// We cannot disprove that the RHS is a PK.
if (result == null) result = Lists.newArrayList();
result.addAll(fkPkCandidate);
}
}
We iterate through all the "simple" equi join conjuncts on the RHS, multiply their NDVs and check if it close to rhsNumRows. The issue here is that this can result in conservative FK/Pk detection if the equi-join conjuncts are not simple (of the form <slotRef> = <slotRef>)
/**
* Returns a new EqJoinConjunctScanSlots for the given equi-join conjunct or null if
* the given conjunct is not of the form <SlotRef> = <SlotRef> or if the underlying
* table/column of at least one side is missing stats.
*/
public static EqJoinConjunctScanSlots create(Expr eqJoinConjunct) {
if (!Expr.IS_EQ_BINARY_PREDICATE.apply(eqJoinConjunct)) return null;
SlotDescriptor lhsScanSlot = eqJoinConjunct.getChild(0).findSrcScanSlot();
if (lhsScanSlot == null || !hasNumRowsAndNdvStats(lhsScanSlot)) return null;
SlotDescriptor rhsScanSlot = eqJoinConjunct.getChild(1).findSrcScanSlot();
For example, the following query contains a complex equi-join conjunct substr(l.c3, 1, 6) = substr(r.c3, 1,6), so while detecting if the left outer join is an FK/PK, we just check if
NDVs(r.c1) * NDVs(r.c2) ~ r.numRows() which is incorrect. (This happens because EqJoinConjunctScanSlots.create() returns null for any non-simple predicates which are not considered later).
[localhost:21000]> explain select * from test_left l left outer join test_right r on l.c1 = r.c1 and l.c2 = r.c2 and substr(l.c3, 1, 6) = substr(r.c3, 1,6); Query: explain select * from test_left l left outer join test_right r on l.c1 = r.c1 and l.c2 = r.c2 and substr(l.c3, 1, 6) = substr(r.c3, 1,6) +-----------------------------------------------------------------------------------------+ | Explain String | +-----------------------------------------------------------------------------------------+ | Max Per-Host Resource Reservation: Memory=1.95MB Threads=5 | | Per-Host Resource Estimates: Memory=66MB | | | | F02:PLAN FRAGMENT [UNPARTITIONED] hosts=1 instances=1 | | | Per-Host Resources: mem-estimate=0B mem-reservation=0B thread-reservation=1 | | PLAN-ROOT SINK | | | mem-estimate=0B mem-reservation=0B thread-reservation=0 | | | | | 04:EXCHANGE [UNPARTITIONED] | | | mem-estimate=0B mem-reservation=0B thread-reservation=0 | | | tuple-ids=0,1N row-size=94B cardinality=49334767023 | | | in pipelines: 00(GETNEXT) | | | | | F00:PLAN FRAGMENT [RANDOM] hosts=1 instances=1 | | Per-Host Resources: mem-estimate=33.94MB mem-reservation=1.95MB thread-reservation=2 | | 02:HASH JOIN [LEFT OUTER JOIN, BROADCAST] | | | hash predicates: l.c1 = r.c1, l.c2 = r.c2, substr(l.c3, 1, 6) = substr(r.c3, 1, 6) | | | fk/pk conjuncts: none | | | mem-estimate=1.94MB mem-reservation=1.94MB spill-buffer=64.00KB thread-reservation=0 | | | tuple-ids=0,1N row-size=94B cardinality=49334767023 | | | in pipelines: 00(GETNEXT), 01(OPEN) | | | | | |--03:EXCHANGE [BROADCAST] | | | | mem-estimate=0B mem-reservation=0B thread-reservation=0 | | | | tuple-ids=1 row-size=47B cardinality=2510 | | | | in pipelines: 01(GETNEXT) | | | | | | | F01:PLAN FRAGMENT [RANDOM] hosts=1 instances=1 | | | Per-Host Resources: mem-estimate=32.00MB mem-reservation=8.00KB thread-reservation=2 | | | 01:SCAN HDFS [cdh72724.test_right r, RANDOM] | | | partitions=1/1 files=1 size=8B | | | stored statistics: | | | table: rows=2510 size=8B | | | columns: all | | | extrapolated-rows=disabled max-scan-range-rows=2510 | | | mem-estimate=32.00MB mem-reservation=8.00KB thread-reservation=1 | | | tuple-ids=1 row-size=47B cardinality=2510 | | | in pipelines: 01(GETNEXT) | | | | | 00:SCAN HDFS [cdh72724.test_left l, RANDOM] | | partitions=1/1 files=1 size=8B | | stored statistics: | | table: rows=589658570 size=8B | | columns: all | | extrapolated-rows=disabled max-scan-range-rows=589658570 | | mem-estimate=32.00MB mem-reservation=8.00KB thread-reservation=1 | | tuple-ids=0 row-size=47B cardinality=589658570 | | in pipelines: 00(GETNEXT) | +-----------------------------------------------------------------------------------------+ Fetched 48 row(s) in 0.02s
We should either consider NDVs of complex conjuncts (error-prone for obvious reasons) or generally assume a join to be FK/PK until and unless proven otherwise.
Here is the plan if the complex conjunct is relaxed to a simple <slotRef> = <slotRef> type. We can see that it detects the join to be of fk/pk type.
[localhost:21000] cdh72724> explain select * from test_left l left outer join test_right r on l.c1 = r.c1 and l.c2 = r.c2 and l.c3 = r.c3; Query: explain select * from test_left l left outer join test_right r on l.c1 = r.c1 and l.c2 = r.c2 and l.c3 = r.c3 +-----------------------------------------------------------------------------------------+ | Explain String | +-----------------------------------------------------------------------------------------+ | Max Per-Host Resource Reservation: Memory=1.95MB Threads=5 | | Per-Host Resource Estimates: Memory=66MB | | | | F02:PLAN FRAGMENT [UNPARTITIONED] hosts=1 instances=1 | | | Per-Host Resources: mem-estimate=0B mem-reservation=0B thread-reservation=1 | | PLAN-ROOT SINK | | | mem-estimate=0B mem-reservation=0B thread-reservation=0 | | | | | 04:EXCHANGE [UNPARTITIONED] | | | mem-estimate=0B mem-reservation=0B thread-reservation=0 | | | tuple-ids=0,1N row-size=94B cardinality=589658570 | | | in pipelines: 00(GETNEXT) | | | | | F00:PLAN FRAGMENT [RANDOM] hosts=1 instances=1 | | Per-Host Resources: mem-estimate=33.94MB mem-reservation=1.95MB thread-reservation=2 | | 02:HASH JOIN [LEFT OUTER JOIN, BROADCAST] | | | hash predicates: l.c1 = r.c1, l.c2 = r.c2, l.c3 = r.c3 | | | fk/pk conjuncts: l.c1 = r.c1, l.c2 = r.c2, l.c3 = r.c3 | | | mem-estimate=1.94MB mem-reservation=1.94MB spill-buffer=64.00KB thread-reservation=0 | | | tuple-ids=0,1N row-size=94B cardinality=589658570 | | | in pipelines: 00(GETNEXT), 01(OPEN) | | | | | |--03:EXCHANGE [BROADCAST] | | | | mem-estimate=0B mem-reservation=0B thread-reservation=0 | | | | tuple-ids=1 row-size=47B cardinality=2510 | | | | in pipelines: 01(GETNEXT) | | | | | | | F01:PLAN FRAGMENT [RANDOM] hosts=1 instances=1 | | | Per-Host Resources: mem-estimate=32.00MB mem-reservation=8.00KB thread-reservation=2 | | | 01:SCAN HDFS [cdh72724.test_right r, RANDOM] | | | partitions=1/1 files=1 size=8B | | | stored statistics: | | | table: rows=2510 size=8B | | | columns: all | | | extrapolated-rows=disabled max-scan-range-rows=2510 | | | mem-estimate=32.00MB mem-reservation=8.00KB thread-reservation=1 | | | tuple-ids=1 row-size=47B cardinality=2510 | | | in pipelines: 01(GETNEXT) | | | | | 00:SCAN HDFS [cdh72724.test_left l, RANDOM] | | partitions=1/1 files=1 size=8B | | stored statistics: | | table: rows=589658570 size=8B | | columns: all | | extrapolated-rows=disabled max-scan-range-rows=589658570 | | mem-estimate=32.00MB mem-reservation=8.00KB thread-reservation=1 | | tuple-ids=0 row-size=47B cardinality=589658570 | | in pipelines: 00(GETNEXT) | +-----------------------------------------------------------------------------------------+ Fetched 48 row(s) in 0.01s
Attachments
Issue Links
- is broken by
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IMPALA-5547 Improve join cardinality estimation with a more robust FK/PK detection
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- Resolved
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- is related to
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- Resolved
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