[jira] [Created] (DRILL-5254) Enhance default reduction factors in optimizer

["Paul Rogers (JIRA)" <ji...@apache.org>]

Paul Rogers created DRILL-5254:
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             Summary: Enhance default reduction factors in optimizer
                 Key: DRILL-5254
                 URL: https://issues.apache.org/jira/browse/DRILL-5254
             Project: Apache Drill
          Issue Type: Improvement
    Affects Versions: 1.9.0
            Reporter: Paul Rogers
            Assignee: Paul Rogers
             Fix For: 1.10


Drill uses Calcite for query parsing and optimization. Drill uses Calcite's default selectivity (reduction factor) rules to compute the number of rows removed by a filter.

The default rules appear to be overly aggressive in estimating reductions. In a production use case, an input with 4 billion rows was estimated to return just 40K rows from a filter. That is, the filter estimated a 1/1,000,000 reduction in rows. As it turns out, the actual reduction was closer to 1/2.

The result was that the planner compared the expected 40K rows against another input of 2.5 million rows, and decided the 40K rows would be best on the build side of a hash join. When confronted with the actual 3 billion rows, the hash join ran out of memory.

The moral of the story is that, in Drill, it is worth being conservative when planning for memory-intensive operations.

The (sanitized) filter is the following, annotated with (a guess at) the default reduction factors in each term:

{code}
col1_s20 in ('Value1','Value2','Value3','Value4',
                     'Value5','Value6','Value7','Value8','Value9') -- 25%
AND col2_i <=3 -- 25%
AND col3_s1 = 'Y' -- 15%
AND col4_s1 = 'Y' -- 15%
AND col5_s6 not like '%str1%' -- 25%
AND col5_s6 not like '%str2%' -- 25%
AND col5_s6 not like '%str3%' -- 25%
AND col5_s6 not like '%str4%' -- 25%
{code}

Total reduction is:

{code}
.25 * .25 * .15 ^ 2 * .25 ^ 4 = 0.000005
{code}

Filter estimation is a known hard problem. In general, one needs statistics and other data, and even then the estimates are just guesses.

Still it is possible to ensure that the defaults are at least unbiased. That is if we assume that the probability of A LIKE B being 25%, then the probability of A NOT LIKE B should be 75%, not also 25%.

This JIRA suggests creating an experimental set of defaults based on the "core" Calcite defaults, but with other reduction factors derived using the laws of probability. In particular:

|| Operator || Revised || Explanation || Calcite Default
| = | 0.15 | Default in Calcite | 0.15
| <> | 0.85 | 1 - p(=) | 0.5
| < | 0.425 | p(<>) / 2 | 0.5
| > | 0.425 | p(<>) / 2 | 0.5
| <= | 0.575 | p(<) + p(=) | 0.5
| >= | 0.575 | p(>) + p(=) | 0.5
| LIKE | 0.25 | Default in Calcite | 0.25
| NOT LIKE | 0.75 | 1 - p(LIKE) | 0.25
| NOT NULL | 0.90 | Default in Calcite | 0.90
| IS NULL | 0.10 | 1 - p(NOT NULL) | 0.25
| IS TRUE | 0.5 | 1 / 2 | 0.25
| IS FALSE | 0.5 | 1 / 2 | 0.25
| IS NOT TRUE | 0.55 | 1 - p(IS TRUE) - p(IS NULL) | .25
| IS NOT FALSE | 0.55 | 1 - p(IS FALSE) - p(IS NULL) | .25
| A OR B | Varies | min(p(A) + p(B) - p(A ^ B), 0.5) | 0.5
| IN (a) | 0.15 | p(=) | 0.5
| x IN (a, b, c, ...) | Varies | p(x = a v x = b v x = c v ...) | 0.5

The Calcite defaults should be taken as approximate.

The probability of the IS NOT TRUE statement assumes the presence of nulls, while IS TRUE does not. The rule for OR caps the reduction factor at 0.5 per standard practice.

With the revised rules, the example WHERE reduction becomes:

{code}
col1_s20 in ('Value1','Value2','Value3','Value4',
                     'Value5','Value6','Value7','Value8','Value9') -- 50%
AND col2_i <=3 -- 57%
AND col3_s1 = 'Y' -- 15%
AND col4_s1 = 'Y' -- 15%
AND col5_s6 not like '%str1%' -- 85%
AND col5_s6 not like '%str2%' -- 85%
AND col5_s6 not like '%str3%' -- 85%
AND col5_s6 not like '%str4%' -- 85%

.5 * .57 * .15^2 * .85^4 = 0.003
{code}

The new rules are not a panacea: they are still just guesses. However, they are unbiased guesses based on the rules of probability which result in more conservative reductions of filters. The result may be better plans in queries with large conjunctions (large number of expressions AND'ed together.)



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