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56 lines
2.0 KiB
Plaintext
56 lines
2.0 KiB
Plaintext
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Integer aggregator/enumerator.
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Many database systems have the notion of a one to many table.
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A one to many table usually sits between two indexed tables,
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as:
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create table one_to_many(left int, right int) ;
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And it is used like this:
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SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right)
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WHERE one_to_many.left = item;
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This will return all the items in the right hand table for an entry
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in the left hand table. This is a very common construct in SQL.
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Now, this methodology can be cumbersome with a very large number of
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entries in the one_to_many table. Depending on the order in which
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data was entered, a join like this could result in an index scan
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and a fetch for each right hand entry in the table for a particular
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left hand entry.
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If you have a very dynamic system, there is not much you can do.
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However, if you have some data which is fairly static, you can
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create a summary table with the aggregator.
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CREATE TABLE summary as SELECT left, int_array_aggregate(right)
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AS right FROM one_to_many GROUP BY left;
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This will create a table with one row per left item, and an array
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of right items. Now this is pretty useless without some way of using
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the array, thats why there is an array enumerator.
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SELECT left, int_array_enum(right) FROM summary WHERE left = item;
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The above query using int_array_enum, produces the same results as:
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SELECT left, right FROM one_to_many WHERE left = item;
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The difference is that the query against the summary table has to get
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only one row from the table, where as the query against "one_to_many"
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must index scan and fetch a row for each entry.
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On our system, an EXPLAIN shows a query with a cost of 8488 gets reduced
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to a cost of 329. The query is a join between the one_to_many table,
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select right, count(right) from
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(
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select left, int_array_enum(right) as right from summary join
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(select left from left_table where left = item) as lefts
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ON (summary.left = lefts.left )
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) as list group by right order by count desc ;
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