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Posted to issues-all@impala.apache.org by "Paul Rogers (JIRA)" <ji...@apache.org> on 2018/10/02 19:00:00 UTC
[jira] [Comment Edited] (IMPALA-7564) Conservative FK/PK join type
detection with complex equi-join conjuncts
[ https://issues.apache.org/jira/browse/IMPALA-7564?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16635969#comment-16635969 ]
Paul Rogers edited comment on IMPALA-7564 at 10/2/18 6:59 PM:
--------------------------------------------------------------
In an RDBMS, the PK is hinted by a unique index. But, Hive has no indexes, so that hint is not available. (The closest are partition keys, but these are not unique. HMS does have some kind of index concept; need to look into that more.) Other DB's can add extra metadata to spell out the ER relationships.
Best would be if HMS identified the PK, FK, and the relationships. A quick glance at the [HMS ER diagram|https://community.hortonworks.com/storage/attachments/7748-hive-ms-121.png] suggests that this information is not available. So, one long-term solution is to add it. Doing so eliminates the need to guess.
Since this case uses a function as part of the key, another help would be if HMS can defined computed (AKA "virtual") columns. Then, that computed column could be identified as a key in the ER relationship.
With all of that, Impala could unambiguously know the user's intent without having to guess.
was (Author: paul.rogers):
In an RDBMS, the PK is hinted by a unique index. But, Hive has no indexes, so that hint is not available. Other DB's can add extra metadata to spell out the ER relationships.
Best would be if HMS identified the PK, FK, and the relationships. A quick glance at the [HMS ER diagram|https://community.hortonworks.com/storage/attachments/7748-hive-ms-121.png] suggests that this information is not available. So, one long-term solution is to add it. Doing so eliminates the need to guess.
Since this case uses a function as part of the key, another help would be if HMS can defined computed (AKA "virtual") columns. Then, that computed column could be identified as a key in the ER relationship.
With all of that, Impala could unambiguously know the user's intent without having to guess.
> Conservative FK/PK join type detection with complex equi-join conjuncts
> -----------------------------------------------------------------------
>
> Key: IMPALA-7564
> URL: https://issues.apache.org/jira/browse/IMPALA-7564
> Project: IMPALA
> Issue Type: Bug
> Components: Frontend
> Affects Versions: Impala 2.12.0, Impala 2.13.0, Impala 3.1.0
> Reporter: bharath v
> Priority: Major
>
> With IMPALA-5547, we predict whether a join is an FK/PK join as follows.
> {noformat}
> // 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);
> }
> }
> {noformat}
> 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>)
> {noformat}
> /**
> * 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();
> {noformat}
> 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).
> {noformat}
> [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
> {noformat}
> 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.
> {noformat}
> [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
> {noformat}
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