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opt: add more cost for lookup joins with more ON conditions
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This is a very limited fix for cockroachdb#34810.

The core problem is that we don't take into account that if we have an
ON condition, not only there's a cost to evaluate it on each row, but
we are generating more internal rows to get a given number of output
rows.

I attempted to do a more general fix (for all join types), where I
tried to estimate the "internal" number of rows using
`unknownFilterSelectivity` for each ON condition. There were two
problems:
 - in some cases (especially with lookup joins) we have an extra ON
   condition that doesn't actually do anything:
     `ab JOIN xy ON a=x AND a=10` becomes
     `ab JOIN xy ON a=x AND a=10 AND x=10` becomes
   and `a=10` could remain as an ON condition. This results in bad
   query plans in important cases (e.g. TPCC) where it prefers to do
   an extra lookup join (due to a non-covering index) just because of
   that condition.

 - we don't have the equality columns readily available for hash join
   (and didn't want to extract them each time we cost). In the future
   we may split the planning into a logical and physical stage, and we
   should then separate the logical joins from hash join.

For 19.1, we simply simply add a cost for lookup joins that is
proportional to the number of remaining ON conditions. This is the
least disruptive method that still fixes the case observed in cockroachdb#34810.
I will leave the issue open to address this properly in the next
release.

Note that although hash joins and merge joins have the same issue in
principle, in practice we always generate these expressions with
equality on all possible columns.

Release note: None
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@RaduBerinde
RaduBerinde committed Mar 11, 2019
1 parent 52cb4ba commit ceba034
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Showing 7 changed files with 193 additions and 11 deletions.
2 changes: 1 addition & 1 deletion pkg/sql/opt/optgen/exprgen/testdata/join
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Expand Up @@ -66,7 +66,7 @@ left-join (lookup abc@ab)
├── columns: t.public.abc.a:5(int) t.public.abc.b:6(int)
├── key columns: [5] = [5]
├── stats: [rows=333333.333]
├── cost: 355060.02
├── cost: 358393.353
├── scan t.public.def
│ ├── columns: t.public.def.d:1(int) t.public.def.e:2(int)
│ ├── stats: [rows=1000]
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27 changes: 27 additions & 0 deletions pkg/sql/opt/xform/coster.go
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Expand Up @@ -363,6 +363,33 @@ func (c *coster) computeLookupJoinCost(join *memo.LookupJoinExpr) memo.Cost {
// cost of emitting the rows.
numLookupCols := join.Cols.Difference(join.Input.Relational().OutputCols).Len()
perRowCost := seqIOCostFactor + c.rowScanCost(join.Table, join.Index, numLookupCols)

// Add a cost if we have to evaluate an ON condition on every row. The more
// leftover conditions, the more expensive it should be. We want to
// differentiate between two lookup joins where one uses only a subset of the
// columns. For example:
// abc JOIN xyz ON a=x AND b=y
// We could have a lookup join using an index on y (and left-over condition
// a=x), and another lookup join on an index on x,y. The latter is definitely
// preferable (the former could generate a lot of internal results that are
// then discarded).
//
// TODO(radu): we should take into account that the "internal" row count is
// higher, according to the selectivities of the conditions. Unfortunately
// this is very tricky, in particular because of left-over conditions that are
// not selective.
// For example:
// ab JOIN xy ON a=x AND x=10
// becomes (during normalization):
// ab JOIN xy ON a=x AND a=10 AND x=10
// which can become a lookup join with left-over condition x=10 which doesn't
// actually filter anything.
//
// TODO(radu): this should be extended to all join types. It's tricky for hash
// joins where we don't have the equality and leftover filters readily
// available.
perRowCost += cpuCostFactor * memo.Cost(len(join.On))

cost += memo.Cost(join.Relational().Stats.RowCount) * perRowCost
return cost
}
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155 changes: 155 additions & 0 deletions pkg/sql/opt/xform/testdata/coster/join
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Expand Up @@ -166,3 +166,158 @@ index-join abc
├── cost: 10.306
├── key: (1)
└── fd: ()-->(3)

# Regression test for #34810: make sure we pick the lookup join that uses
# all equality columns.

exec-ddl
CREATE TABLE abcde (
a TEXT NOT NULL,
b UUID NOT NULL,
c UUID NOT NULL,
d VARCHAR(255) NOT NULL,
e TEXT NOT NULL,
CONSTRAINT "primary" PRIMARY KEY (a, b, c),
UNIQUE INDEX idx_abd (a, b, d),
UNIQUE INDEX idx_abcd (a, b, c, d)
)
----
TABLE abcde
├── a string not null
├── b uuid not null
├── c uuid not null
├── d string not null
├── e string not null
├── INDEX primary
│ ├── a string not null
│ ├── b uuid not null
│ └── c uuid not null
├── INDEX idx_abd
│ ├── a string not null
│ ├── b uuid not null
│ ├── d string not null
│ └── c uuid not null (storing)
└── INDEX idx_abcd
├── a string not null
├── b uuid not null
├── c uuid not null
└── d string not null

exec-ddl
ALTER TABLE abcde INJECT STATISTICS '[
{
"columns": ["a"],
"created_at": "2019-02-08 04:10:40.001179+00:00",
"row_count": 250000,
"distinct_count": 1
},
{
"columns": ["b"],
"created_at": "2019-02-08 04:10:40.119954+00:00",
"row_count": 250000,
"distinct_count": 2
},
{
"columns": ["d"],
"created_at": "2019-02-08 04:10:40.119954+00:00",
"row_count": 250000,
"distinct_count": 125000
}
]'
----

exec-ddl
CREATE TABLE wxyz (
w TEXT NOT NULL,
x UUID NOT NULL,
y UUID NOT NULL,
z TEXT NOT NULL,
CONSTRAINT "primary" PRIMARY KEY (w, x, y),
CONSTRAINT "foreign" FOREIGN KEY (w, x, y) REFERENCES abcde (a, b, c)
)
----
TABLE wxyz
├── w string not null
├── x uuid not null
├── y uuid not null
├── z string not null
├── INDEX primary
│ ├── w string not null
│ ├── x uuid not null
│ └── y uuid not null
└── FOREIGN KEY (w, x, y) REFERENCES t.public.abcde (a, b, c)

exec-ddl
ALTER TABLE wxyz INJECT STATISTICS '[
{
"columns": ["w"],
"created_at": "2019-02-08 04:10:40.001179+00:00",
"row_count": 10000,
"distinct_count": 1
},
{
"columns": ["x"],
"created_at": "2019-02-08 04:10:40.119954+00:00",
"row_count": 10000,
"distinct_count": 1
},
{
"columns": ["y"],
"created_at": "2019-02-08 04:10:40.119954+00:00",
"row_count": 10000,
"distinct_count": 2500
}
]'
----

opt
SELECT w, x, y, z
FROM wxyz
INNER JOIN abcde
ON w = a AND x = b AND y = c
WHERE w = 'foo' AND x = '2AB23800-06B1-4E19-A3BB-DF3768B808D2'
ORDER BY d
LIMIT 10
----
project
├── columns: w:1(string!null) x:2(uuid!null) y:3(uuid!null) z:4(string!null) [hidden: d:8(string!null)]
├── cardinality: [0 - 10]
├── stats: [rows=10]
├── cost: 122481.301
├── key: (8)
├── fd: ()-->(1,2), (3)-->(4,8), (8)-->(3,4)
├── ordering: +8 opt(1,2) [actual: +8]
└── limit
├── columns: w:1(string!null) x:2(uuid!null) y:3(uuid!null) z:4(string!null) a:5(string!null) b:6(uuid!null) c:7(uuid!null) d:8(string!null)
├── internal-ordering: +8 opt(1,2,5,6)
├── cardinality: [0 - 10]
├── stats: [rows=10]
├── cost: 122481.191
├── key: (7)
├── fd: ()-->(1,2,5,6), (3)-->(4), (7)-->(8), (8)-->(7), (1)==(5), (5)==(1), (2)==(6), (6)==(2), (3)==(7), (7)==(3)
├── ordering: +8 opt(1,2,5,6) [actual: +8]
├── sort
│ ├── columns: w:1(string!null) x:2(uuid!null) y:3(uuid!null) z:4(string!null) a:5(string!null) b:6(uuid!null) c:7(uuid!null) d:8(string!null)
│ ├── stats: [rows=50048.8759, distinct(1)=1, null(1)=0, distinct(2)=1, null(2)=0, distinct(3)=2500, null(3)=0, distinct(4)=1000, null(4)=0, distinct(5)=1, null(5)=0, distinct(6)=1, null(6)=0, distinct(7)=2500, null(7)=0, distinct(8)=38781.1698, null(8)=0]
│ ├── cost: 122481.081
│ ├── key: (7)
│ ├── fd: ()-->(1,2,5,6), (3)-->(4), (7)-->(8), (8)-->(7), (1)==(5), (5)==(1), (2)==(6), (6)==(2), (3)==(7), (7)==(3)
│ ├── ordering: +8 opt(1,2,5,6) [actual: +8]
│ └── inner-join (lookup abcde@idx_abcd)
│ ├── columns: w:1(string!null) x:2(uuid!null) y:3(uuid!null) z:4(string!null) a:5(string!null) b:6(uuid!null) c:7(uuid!null) d:8(string!null)
│ ├── key columns: [1 2 3] = [5 6 7]
│ ├── stats: [rows=50048.8759, distinct(1)=1, null(1)=0, distinct(2)=1, null(2)=0, distinct(3)=2500, null(3)=0, distinct(4)=1000, null(4)=0, distinct(5)=1, null(5)=0, distinct(6)=1, null(6)=0, distinct(7)=2500, null(7)=0, distinct(8)=38781.1698, null(8)=0]
│ ├── cost: 105853.783
│ ├── key: (7)
│ ├── fd: ()-->(1,2,5,6), (3)-->(4), (7)-->(8), (8)-->(7), (1)==(5), (5)==(1), (2)==(6), (6)==(2), (3)==(7), (7)==(3)
│ ├── scan wxyz
│ │ ├── columns: w:1(string!null) x:2(uuid!null) y:3(uuid!null) z:4(string!null)
│ │ ├── constraint: /1/2/3: [/'foo'/'2ab23800-06b1-4e19-a3bb-df3768b808d2' - /'foo'/'2ab23800-06b1-4e19-a3bb-df3768b808d2']
│ │ ├── stats: [rows=10000, distinct(1)=1, null(1)=0, distinct(2)=1, null(2)=0, distinct(3)=2500, null(3)=0, distinct(4)=1000, null(4)=0]
│ │ ├── cost: 10800.01
│ │ ├── key: (3)
│ │ └── fd: ()-->(1,2), (3)-->(4)
│ └── filters
│ ├── a = 'foo' [type=bool, outer=(5), constraints=(/5: [/'foo' - /'foo']; tight), fd=()-->(5)]
│ └── b = '2ab23800-06b1-4e19-a3bb-df3768b808d2' [type=bool, outer=(6), constraints=(/6: [/'2ab23800-06b1-4e19-a3bb-df3768b808d2' - /'2ab23800-06b1-4e19-a3bb-df3768b808d2']; tight), fd=()-->(6)]
└── const: 10 [type=int]
4 changes: 2 additions & 2 deletions pkg/sql/opt/xform/testdata/coster/zone
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Expand Up @@ -317,7 +317,7 @@ inner-join (lookup xy@y2)
├── flags: no-merge-join;no-hash-join
├── key columns: [2] = [5]
├── stats: [rows=98.01, distinct(1)=9.9, null(1)=0, distinct(2)=1, null(2)=0, distinct(4)=9.9, null(4)=0, distinct(5)=1, null(5)=0]
├── cost: 152.0444
├── cost: 153.0245
├── key: (1,4)
├── fd: ()-->(2,5), (1)-->(3), (2,3)~~>(1), (2)==(5), (5)==(2)
├── prune: (1,3,4)
Expand Down Expand Up @@ -359,7 +359,7 @@ inner-join (lookup xy@y1)
├── flags: no-merge-join;no-hash-join
├── key columns: [2] = [5]
├── stats: [rows=98.01, distinct(1)=9.9, null(1)=0, distinct(2)=1, null(2)=0, distinct(4)=9.9, null(4)=0, distinct(5)=1, null(5)=0]
├── cost: 152.0444
├── cost: 153.0245
├── key: (1,4)
├── fd: ()-->(2,5), (1)-->(3), (2,3)~~>(1), (2)==(5), (5)==(2)
├── prune: (1,3,4)
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4 changes: 2 additions & 2 deletions pkg/sql/opt/xform/testdata/external/tpcc
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Expand Up @@ -904,15 +904,15 @@ scalar-group-by
├── columns: count:28(int)
├── cardinality: [1 - 1]
├── stats: [rows=1]
├── cost: 1.82111111
├── cost: 1.84111111
├── key: ()
├── fd: ()-->(28)
├── prune: (28)
├── inner-join (lookup stock)
│ ├── columns: ol_o_id:1(int!null) ol_d_id:2(int!null) ol_w_id:3(int!null) ol_i_id:5(int!null) s_i_id:11(int!null) s_w_id:12(int!null) s_quantity:13(int!null)
│ ├── key columns: [3 5] = [12 11]
│ ├── stats: [rows=1, distinct(1)=0.11109736, null(1)=0, distinct(2)=0.111097416, null(2)=0, distinct(3)=0.111111111, null(3)=0, distinct(5)=0.111111056, null(5)=0, distinct(11)=0.111111056, null(11)=0, distinct(12)=0.111111111, null(12)=0, distinct(13)=1, null(13)=0]
│ ├── cost: 1.79111111
│ ├── cost: 1.81111111
│ ├── fd: ()-->(2,3,12), (11)-->(13), (5)==(11), (11)==(5), (3)==(12), (12)==(3)
│ ├── interesting orderings: (+3,+2,-1)
│ ├── scan order_line
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8 changes: 4 additions & 4 deletions pkg/sql/opt/xform/testdata/external/tpcc-no-stats
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Expand Up @@ -709,15 +709,15 @@ scalar-group-by
├── columns: count:28(int)
├── cardinality: [1 - 1]
├── stats: [rows=1]
├── cost: 0.141477778
├── cost: 0.142211111
├── key: ()
├── fd: ()-->(28)
├── prune: (28)
├── inner-join (lookup stock)
│ ├── columns: ol_o_id:1(int!null) ol_d_id:2(int!null) ol_w_id:3(int!null) ol_i_id:5(int!null) s_i_id:11(int!null) s_w_id:12(int!null) s_quantity:13(int!null)
│ ├── key columns: [3 5] = [12 11]
│ ├── stats: [rows=0.0366666667, distinct(1)=0.0111105556, null(1)=0, distinct(2)=0.0111111111, null(2)=0, distinct(3)=0.0111111111, null(3)=0, distinct(5)=0.0111105556, null(5)=0, distinct(11)=0.0111105556, null(11)=0, distinct(12)=0.0111111111, null(12)=0, distinct(13)=0.0366666667, null(13)=0]
│ ├── cost: 0.121111111
│ ├── cost: 0.121844444
│ ├── fd: ()-->(2,3,12), (11)-->(13), (5)==(11), (11)==(5), (3)==(12), (12)==(3)
│ ├── interesting orderings: (+3,+2,-1)
│ ├── scan order_line
Expand Down Expand Up @@ -762,15 +762,15 @@ scalar-group-by
├── columns: count:22(int)
├── cardinality: [1 - 1]
├── stats: [rows=1]
├── cost: 1588.01
├── cost: 1588.34
├── key: ()
├── fd: ()-->(22)
├── prune: (22)
├── inner-join (lookup warehouse)
│ ├── columns: w_id:1(int!null) w_ytd:9(decimal!null) d_w_id:11(int!null) sum:21(decimal!null)
│ ├── key columns: [11] = [1]
│ ├── stats: [rows=33, distinct(1)=33, null(1)=0, distinct(9)=28.3508504, null(9)=0, distinct(11)=33, null(11)=0, distinct(21)=28.3508504, null(21)=0]
│ ├── cost: 1587.66
│ ├── cost: 1587.99
│ ├── key: (11)
│ ├── fd: (1)-->(9), (11)-->(21), (1)==(11), (11)==(1)
│ ├── interesting orderings: (+11)
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4 changes: 2 additions & 2 deletions pkg/sql/opt/xform/testdata/rules/join
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Expand Up @@ -1879,11 +1879,11 @@ memo (optimized, ~11KB, required=[presentation: a:1])
├── G1: (project G2 G3 a)
│ └── [presentation: a:1]
│ ├── best: (project G2 G3 a)
│ └── cost: 87.93
│ └── cost: 88.05
├── G2: (select G4 G5) (lookup-join G6 G5 t5,keyCols=[1],outCols=(1,2)) (select G7 G5)
│ └── []
│ ├── best: (lookup-join G6 G5 t5,keyCols=[1],outCols=(1,2))
│ └── cost: 87.80
│ └── cost: 87.92
├── G3: (projections)
├── G4: (scan t5,cols=(1,2))
│ └── []
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