tanelk opened a new pull request #32075:
URL: https://github.com/apache/spark/pull/32075
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### What changes were proposed in this pull request?
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Changed the cost comparison function of the CBO to use the ratios of row
counts and sizes in bytes.
### Why are the changes needed?
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In #30965 we changed to CBO cost comparison function so it would be
"symetric": `A.betterThan(B)` now implies, that `!B.betterThan(A)`.
With that we caused a performance regressions in some queries - TPCDS q19
for example.
The original cost comparison function used the ratios `relativeRows =
A.rowCount / B.rowCount` and `relativeSize = A.size / B.size`. The changed
function compared "absolute" cost values `costA = w*A.rowCount + (1-w)*A.size`
and `costB = w*B.rowCount + (1-w)*B.size`.
Given the input from @wzhfy we decided to go back to the relative values,
because otherwise one (size) may overwhelm the other (rowCount). But this time
we avoid adding up the ratios.
Originally `A.betterThan(B) => w*relativeRows + (1-w)*relativeSize < 1` was
used. Besides being "non-symteric", this also can exhibit one overwhelming
other.
For `w=0.5` If `A` size (bytes) is at least 2x larger than `B`, then no
matter how many times more rows does the `B` plan have, `B` will allways be
considered to be better - `0.5*2 + 0.5*0.00000000000001 > 1`.
When working with ratios, then it would be better to multiply them.
The proposed cost comparison function is: `A.betterThan(B) => relativeRows^w
* relativeSize^(1-w) < 1`.
### Does this PR introduce _any_ user-facing change?
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Comparison of the changed TPCDS v1.4 query execution times at sf=10:
| absolute | multiplicative | | additive |
-- | -- | -- | -- | -- | --
q12 | 145 | 137 | -5.52% | 141 | -2.76%
q13 | 264 | 271 | 2.65% | 271 | 2.65%
q17 | 4521 | 4243 | -6.15% | 4348 | -3.83%
q18 | 758 | 466 | -38.52% | 480 | -36.68%
q19 | 38503 | 2167 | -94.37% | 2176 | -94.35%
q20 | 119 | 120 | 0.84% | 126 | 5.88%
q24a | 16429 | 16838 | 2.49% | 17103 | 4.10%
q24b | 16592 | 16999 | 2.45% | 17268 | 4.07%
q25 | 3558 | 3556 | -0.06% | 3675 | 3.29%
q33 | 362 | 361 | -0.28% | 380 | 4.97%
q52 | 1020 | 1032 | 1.18% | 1052 | 3.14%
q55 | 927 | 938 | 1.19% | 961 | 3.67%
q72 | 24169 | 13377 | -44.65% | 24306 | 0.57%
q81 | 1285 | 1185 | -7.78% | 1168 | -9.11%
q91 | 324 | 336 | 3.70% | 337 | 4.01%
q98 | 126 | 129 | 2.38% | 131 | 3.97%
All times are in ms, the change is compared to the situation in the master
branch (absolute).
The proposed cost function (multiplicative) significantlly improves the
performance on q18, q19 and q72. The original cost function (additive) has
similar improvements at q18 and q19. All other chagnes are within the error
bars and I would ignore them - perhaps q81 has also improved.
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PlanStabilitySuite
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