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POEfficiently selecting from many-to-many relation in H2
 

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  1. POEfficiently selecting from many-to-many relation in H2
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    <p>I'm using H2, and I have a database of books (table Entries) and authors (table Persons), connected through a many-to-many relationship, itself stored in a table Authorship. The database is fairly large (900'000+ persons and 2.5M+ books).</p> <p>I'm trying to efficiently select the list of all books authored by at least one author whose name matches a pattern (LIKE '%pattern%'). The trick here is that the pattern should severly restrict the number of matching authors, and each author has a reasonably small number of associated books.</p> <p>I tried two queries:</p> <pre><code>SELECT p.*, e.title FROM (SELECT * FROM Persons WHERE name LIKE '%pattern%') AS p INNER JOIN Authorship AS au ON au.authorId = p.id INNER JOIN Entries AS e ON e.id = au.entryId; </code></pre> <p>and:</p> <pre><code>SELECT p.*, e.title FROM Persons AS p INNER JOIN Authorship AS au ON au.authorId = p.id INNER JOIN Entries AS e ON e.id = au.entryId WHERE p.name like '%pattern%'; </code></pre> <p>I expected the first one to be much faster, as I'm joining a much smaller (sub)table of authors, however they both take as long. So long in fact that I can manually decompose the query into three selects and find the result I want faster.</p> <p>When I try to EXPLAIN the queries, I observe that indeed they are very similar (a full join on the tables and only then a WHERE clause), so my question is: how can I achieve a fast select, that relies on the fact that the filter on authors should result in a much smaller join with the other two tables?</p> <p>Note that I tried the same queries with MySQL and got results in line with what I expected (selecting first is much faster).</p> <p>Thank you.</p>
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    1. USPhilippe
    1. USPhilippe
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    1. COWhy that SUBSELECT in the first JOIN? Why not simply `INNER JOIN Authorship AS au ON ... `?
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      1. USwonk0
    2. COYou're right, I made the change. The queries translate to the same thing (according to EXPLAIN at least) but are now simpler, thanks.
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      1. USPhilippe
    3. COWhat is the result of `EXPLAIN ANALYZE SELECT ...` for those queries?
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      1. USThomas Mueller
 

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