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module ArJdbc
module PostgreSQL
# @private
module ExplainSupport
def supports_explain?
true
end
def explain(arel, binds = [])
sql = "EXPLAIN #{to_sql(arel, binds)}"
result = exec_query(sql, "EXPLAIN", binds)
ExplainPrettyPrinter.new.pp result # we can assume AR >= 3.1
end
# @private
class ExplainPrettyPrinter
# Pretty prints the result of a EXPLAIN in a way that resembles the output of the
# PostgreSQL shell:
#
# QUERY PLAN
# ------------------------------------------------------------------------------
# Nested Loop Left Join (cost=0.00..37.24 rows=8 width=0)
# Join Filter: (posts.user_id = users.id)
# -> Index Scan using users_pkey on users (cost=0.00..8.27 rows=1 width=4)
# Index Cond: (id = 1)
# -> Seq Scan on posts (cost=0.00..28.88 rows=8 width=4)
# Filter: (posts.user_id = 1)
# (6 rows)
#
def pp(result)
header = result.columns.first
lines = result.rows.map(&:first)
# We add 2 because there's one char of padding at both sides, note
# the extra hyphens in the example above.
width = [header, *lines].map(&:length).max + 2
pp = []
pp << header.center(width).rstrip
pp << '-' * width
pp += lines.map {|line| " #{line}"}
nrows = result.rows.length
rows_label = nrows == 1 ? 'row' : 'rows'
pp << "(#{nrows} #{rows_label})"
pp.join("\n") + "\n"
end
end
end
end
end
Version data entries
19 entries across 19 versions & 2 rubygems