1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
|
# https://www.periscopedata.com/blog/medians-in-sql.html
module Gitlab
module Database
module Median
NotSupportedError = Class.new(StandardError)
def median_datetime(arel_table, query_so_far, column_sym)
extract_median(execute_queries(arel_table, query_so_far, column_sym)).presence
end
def median_datetimes(arel_table, query_so_far, column_sym, partition_column)
extract_medians(execute_queries(arel_table, query_so_far, column_sym, partition_column)).presence
end
def extract_median(results)
result = results.compact.first
if Gitlab::Database.postgresql?
result = result.first.presence
result['median']&.to_f if result
elsif Gitlab::Database.mysql?
result.to_a.flatten.first
end
end
def extract_medians(results)
median_values = results.compact.first.values
median_values.each_with_object({}) do |(id, median), hash|
hash[id.to_i] = median&.to_f
end
end
def mysql_median_datetime_sql(arel_table, query_so_far, column_sym)
query = arel_table
.from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name))
.project(average([arel_table[column_sym]], 'median'))
.where(
Arel::Nodes::Between.new(
Arel.sql("(select @row_id := @row_id + 1)"),
Arel::Nodes::And.new(
[Arel.sql('@ct/2.0'),
Arel.sql('@ct/2.0 + 1')]
)
)
).
# Disallow negative values
where(arel_table[column_sym].gteq(0))
[
Arel.sql("CREATE TEMPORARY TABLE IF NOT EXISTS #{query_so_far.to_sql}"),
Arel.sql("set @ct := (select count(1) from #{arel_table.table_name});"),
Arel.sql("set @row_id := 0;"),
query.to_sql,
Arel.sql("DROP TEMPORARY TABLE IF EXISTS #{arel_table.table_name};")
]
end
def pg_median_datetime_sql(arel_table, query_so_far, column_sym, partition_column = nil)
# Create a CTE with the column we're operating on, row number (after sorting by the column
# we're operating on), and count of the table we're operating on (duplicated across) all rows
# of the CTE. For example, if we're looking to find the median of the `projects.star_count`
# column, the CTE might look like this:
#
# star_count | row_id | ct
# ------------+--------+----
# 5 | 1 | 3
# 9 | 2 | 3
# 15 | 3 | 3
#
# If a partition column is used we will do the same operation but for separate partitions,
# when that happens the CTE might look like this:
#
# project_id | star_count | row_id | ct
# ------------+------------+--------+----
# 1 | 5 | 1 | 2
# 1 | 9 | 2 | 2
# 2 | 10 | 1 | 3
# 2 | 15 | 2 | 3
# 2 | 20 | 3 | 3
cte_table = Arel::Table.new("ordered_records")
cte = Arel::Nodes::As.new(
cte_table,
arel_table.project(*rank_rows(arel_table, column_sym, partition_column)).
# Disallow negative values
where(arel_table[column_sym].gteq(zero_interval)))
# From the CTE, select either the middle row or the middle two rows (this is accomplished
# by 'where cte.row_id between cte.ct / 2.0 AND cte.ct / 2.0 + 1'). Find the average of the
# selected rows, and this is the median value.
result =
cte_table
.project(*median_projections(cte_table, column_sym, partition_column))
.where(
Arel::Nodes::Between.new(
cte_table[:row_id],
Arel::Nodes::And.new(
[(cte_table[:ct] / Arel.sql('2.0')),
(cte_table[:ct] / Arel.sql('2.0') + 1)]
)
)
)
.with(query_so_far, cte)
result.group(cte_table[partition_column]).order(cte_table[partition_column]) if partition_column
result.to_sql
end
private
def median_queries(arel_table, query_so_far, column_sym, partition_column = nil)
if Gitlab::Database.postgresql?
pg_median_datetime_sql(arel_table, query_so_far, column_sym, partition_column)
elsif Gitlab::Database.mysql?
raise NotSupportedError, "partition_column is not supported for MySQL" if partition_column
mysql_median_datetime_sql(arel_table, query_so_far, column_sym)
end
end
def execute_queries(arel_table, query_so_far, column_sym, partition_column = nil)
queries = median_queries(arel_table, query_so_far, column_sym, partition_column)
Array.wrap(queries).map { |query| ActiveRecord::Base.connection.execute(query) }
end
def average(args, as)
Arel::Nodes::NamedFunction.new("AVG", args, as)
end
def rank_rows(arel_table, column_sym, partition_column)
column_row = arel_table[column_sym].as(column_sym.to_s)
if partition_column
partition_row = arel_table[partition_column]
row_id =
Arel::Nodes::Over.new(
Arel::Nodes::NamedFunction.new('rank', []),
Arel::Nodes::Window.new.partition(arel_table[partition_column])
.order(arel_table[column_sym])
).as('row_id')
arel_from = if Gitlab.rails5?
arel_table.from.from(arel_table.alias)
else
arel_table.from(arel_table.alias)
end
count = arel_from.project('COUNT(*)')
.where(arel_table[partition_column].eq(arel_table.alias[partition_column]))
.as('ct')
[partition_row, column_row, row_id, count]
else
row_id =
Arel::Nodes::Over.new(
Arel::Nodes::NamedFunction.new('row_number', []),
Arel::Nodes::Window.new.order(arel_table[column_sym])
).as('row_id')
count = arel_table.project("COUNT(1)").as('ct')
[column_row, row_id, count]
end
end
def median_projections(table, column_sym, partition_column)
projections = []
projections << table[partition_column] if partition_column
projections << average([extract_epoch(table[column_sym])], "median")
projections
end
def extract_epoch(arel_attribute)
Arel.sql(%Q{EXTRACT(EPOCH FROM "#{arel_attribute.relation.name}"."#{arel_attribute.name}")})
end
def extract_diff_epoch(diff)
return diff unless Gitlab::Database.postgresql?
Arel.sql(%Q{EXTRACT(EPOCH FROM (#{diff.to_sql}))})
end
# Need to cast '0' to an INTERVAL before we can check if the interval is positive
def zero_interval
Arel::Nodes::NamedFunction.new("CAST", [Arel.sql("'0' AS INTERVAL")])
end
end
end
end
|