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https://issues.apache.org/jira/browse/DERBY-4079?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12678313#action_12678313
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Knut Anders Hatlen commented on DERBY-4079:
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I don't have a copy of SQL 2008 yet, so I can't comment on the syntax, but the
docs look good to me. I did however struggle with understanding this sentence:
> If the underlying result set is not materialized (depends on query), the use
> of this clause can give efficiency benefits.
What I don't quite understand, is the implication that we won't see efficiency
benefits if the underlying result set is materialized, but I'm probably just
misunderstanding what you are comparing (or perhaps misunderstanding how you're
planning to implement it).
If you are comparing the time it takes to execute SELECT * FROM LARGE_TABLE and
SELECT * FROM LARGE_TABLE FETCH FIRST 10 ROWS ONLY and fetch the full result, I
understand that it would be faster with the FETCH clause than without if the
underlying result set is not materialized, because you just stop after fetching
the first 10 rows whereas the other query goes through the entire table. If the
underlying result set is materialized, I understand that it's probably not
going to speed it up significantly in embedded mode, since the same number of
rows must be processed in both queries. In a client/server environment it
reduces the number of rows to send over the wire, so there we should see an
efficiency benefit even if the result set is materialized, I believe.
If you are comparing the time it takes to read out the columns you're
interested in (just quit calling rs.next() once you've fetched N rows), I don't
see that it should make much difference at all whether or not OFFSET/FETCH is
used (assuming that the implementation will basically do the same thing, just
on a slightly lower level). Again, in a client/server environment you may see
higher performance with OFFSET/FETCH because the skipped rows don't need to go
over the wire, and because you limit the number of rows that will be
pre-fetched. But I don't see how this benefit will be affected by whether or
not the underlying result set is materialized.
Perhaps it suffices to say "The use of this clause can give efficiency benefits
for some queries" and not qualify it further?
> Add support for SQL:2008 <result offset clause> and <fetch first clause> to
> limit result set cardinality
> --------------------------------------------------------------------------------------------------------
>
> Key: DERBY-4079
> URL: https://issues.apache.org/jira/browse/DERBY-4079
> Project: Derby
> Issue Type: New Feature
> Components: SQL
> Reporter: Dag H. Wanvik
> Assignee: Dag H. Wanvik
> Attachments: ref.zip
>
>
> SQL 2008 has added new syntax to support a direct way to limit the
> returned set of rows in a result set. This allows an application to
> retrieve only some rows of an otherwise larger result set, similar to
> the popular LIMIT clauses use in some databases.
> Up till now, in Derby (and SQL) we have had to use the ROW_NUMBER()
> function in a nested subquery to achieve the effect of the <fetch
> first clause>, cf. DERBY-2998, a method which is rather more indirect
> and still not efficient (DERBY-3505), and primarily intended for OLAP
> functionality, perhaps.
> There has been no direct way to achieve the effect of the <result
> offset clause> via SQL.
> Syntax (cf. SQL 2008, section 7.13):
> <result offset clause> ::= OFFSET <n> {ROW | ROWS}
> <fetch first clause> ::= FETCH {FIRST | NEXT} [<n>] {ROW | ROWS}
> ONLY
> where <n> is an integer. The two clauses syntactically follow the ORDER BY
> clause in the grammar.
> Note that both ORDER BY and the new clauses above are allowed also in
> subqueries in the new version of the SQL standard (section 7.13). I
> only propose to include this at the top level in DERBY for now. (ORDER
> BY is presently also not allowed in subqueries in Derby since SQL
> didn't allow for this until SQL 2008 either).
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