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1. MYSQL JOIN WITH A SUBQUERY MOVIE
2. MYSQL JOIN WITH A SUBQUERY UPDATE
3. MYSQL JOIN WITH A SUBQUERY FULL
4. MYSQL JOIN WITH A SUBQUERY SOFTWARE

MYSQL JOIN WITH A SUBQUERY FULL

Index = full index scan Scanning the entire data set? full index scan > full table scan (covering index) range = partial index scan, >= IS NULL, BETWEEN, IN One row per table.ġ4 type index_subquery using a non-unique index of one table row ******************* id: 1 select_type: SIMPLE table: rental type: const “Data access method” Get this as good as possible One row per table.ġ3 type ALL = full table scan Everything else uses an index row ******************* id: 1 select_type: SIMPLE table: rental - One per table/alias - NULL One row per table.ĮXPLAIN SELECT return_date FROM rental WHERE rental_id=0\G One row per table.ġ2 type “Data access method” Get this as good as possible Used in subqueries More on subqueries later DEPENDENT UNION DEPENDENT SUBQUERY DERIVED UNCACHEABLE SUBQUERY One row per table. row ******************* id: 1 select_type: SIMPLE SIMPLE – one table, or JOINs PRIMARY First SELECT in a UNION Outer query of a subquery UNION, UNION RESULT One row per table. row ******************* id: 1 Id = sequential identifier One per table, subquery, derived table No row returned for a view Because it is virtual Underlying tables are represented One row per table.Ĩ select_type SIMPLE – one table, or JOINs PRIMARY row ******************* id: 1 select_type: SIMPLE table: rental type: const possible_keys: PRIMARY key: PRIMARY key_len: 4 ref: const rows: 1 Extra: 1 row in set (0.00 sec) One row per table.ħ Id Id = sequential identifier One per table, subquery, derived table

InnoDB - approx stats InnoDB - one method of doing dives into the data MyISAM has better/more accurate metadataĦ EXPLAIN Output EXPLAIN returns 10 fields: Possible and actual indexes used Length of index used Approx # of records examinedĥ Metadata Optimizer uses metadata: cardinality, # rows, etc. How data is looked up If there are subqueries, unions, sortsĤ What EXPLAIN Shows If WHERE, DISTINCT are used

MYSQL JOIN WITH A SUBQUERY UPDATE

UPDATE tbl SET fld1=“foo” WHERE fld2=“bar” can be changed to: EXPLAIN SELECT fld1 FROM tbl WHERE fld2=”bar” ģ What EXPLAIN Shows How many tables How tables are joined Sheeri Cabral Senior DB Admin/Architect, Mozilla Northeast PHP 2012Ģ EXPLAIN SQL extension SELECT only Can modify other statements:

With different LIMITs I got different results but at some point when I go over a certain limit, the result is exactly as it is without the limit - but equally wrong.Presentation on theme: "Optimizing MySQL Joins and Subqueries"- Presentation transcript: LIMIT 0, 50000) AS DVDPROFILER_dvd_actor SELECT count( * ) AS Count, lastname, firstname So I toyed a bit and when I introduced a LIMIT clause into the subquery I suddenly got different yet never correct replies. So I checked the subquery with a selected actor SELECT caid, id ON DVDPROFILER_dvd_common_actor.caid = DVDPROFILER_dvd_actor.caid My goal is to extract the information of how many different movies an actor is profiled in and I thought - naive as I am - it should be as simple as this: SELECT count( * ) AS Count, lastname, firstname I get exactly what i'd expect: The top actors counted by the times he's credited in any movie, even if that's multiple times for multiple roles. INNER JOIN DVDPROFILER_dvd_actor ON DVDPROFILER_dvd_common_actor.caid = DVDPROFILER_dvd_actor.caid When I create a full inner join like this: SELECT count( * ) AS Count, lastname, firstname

MYSQL JOIN WITH A SUBQUERY MOVIE

The first one is a table that contains actors, identifier: caid.There's an m:n table construct with 3 tables of which only 2 are relevant to the problem. Database client version: libmysql - 5.5.32.

MYSQL JOIN WITH A SUBQUERY SOFTWARE