SORTING OUT MULTIPLE SIMILAR TABLE IN A MYSQL VIEW - mysql

Question

I all,
i have 2 similar very LARGE table(1M rows each) with the same layout, i would union them and sorting by a common column: start . also i would put a condition in "start" ie : start>X.
the problem is that the view doesnt take care abount start's index and the the complexity rise up much, a simple query takes about 15 seconds and inserting a LIMIT doesnt fix because the results are cutted off first.
CREATE VIEW CDR AS
(SELECT start, duration, clid, FROM cdr_md ORDER BY start LIMIT 1000)
UNION ALL
(SELECT start, duration, clid, FROM cdr_1025 ORDER BY start LIMIT 1000)
ORDER BY start ;
a query to:
SELECT * FROM CDR WHERE start>10
doesnt returns expected results cause LIMIT keyword cuts off results prior.
the expected results would be as a query like this:
CREATE VIEW CDR AS
(SELECT start, duration, clid, FROM cdr_md WHERE start>X ORDER BY start LIMIT 1000)
UNION ALL
(SELECT start, duration, clid, FROM cdr_1025 WHERE start>X ORDER BY start LIMIT 1000)
ORDER BY start ;
Is there a way to avoid this problem ?
Thaks all
Fabrizio

Answer 1

i have 2 similar table ... with the same layout
This is contrary to the Principle of Orthogonal Design.
Don't do it. At least not without very good reason—with suitable indexes, 1 million records per table is easily enough for MySQL to handle without any need for partitioning; and even if one did need to partition the data, there are better ways than this manual kludge (which can give rise to ambiguous, potentially inconsistent data and lead to redundancy and complexity in your data manipulation code).
Instead, consider combining your tables into a single one with suitable columns to distinguish the records' differences. For example:
CREATE TABLE cdr_combined AS
SELECT *, 'md' AS orig FROM cdr_md
UNION ALL
SELECT *, '1025' AS orig FROM cdr_1025
;
DROP TABLE cdr_md, cdr_1025;
If you will always be viewing your data along the previously "partitioned" axis, include the distinguishing columns as index prefixes and performance will generally improve versus having separate tables.
You then won't need to perform any UNION and your VIEW definition effectively becomes:
CREATE VIEW CDR AS
SELECT start, duration, clid, FROM cdr_combined ORDER BY start
However, be aware that queries on views may not always perform as well as using the underlying tables directly. As documented under Restrictions on Views:
View processing is not optimized:
It is not possible to create an index on a view.
Indexes can be used for views processed using the merge algorithm. However, a view that is processed with the temptable algorithm is unable to take advantage of indexes on its underlying tables (although indexes can be used during generation of the temporary tables).