mysql explain结果中Type的含义

摘自：https://dev.mysql.com/doc/workbench/en/wb-performance-explain.html

Table 7.1 Visual Explain Diagram Information

System Name	Color	Text on Visual Diagram	Tooltip related information
SYSTEM	Blue	Single row: system constant	Very low cost
CONST	Blue	Single row: constant	Very low cost
EQ_REF	Green	Unique Key Lookup	Low cost -- The optimizer is able to find an index that it can use to retrieve the required records. It is fast because the index search directly leads to the page with all the row data
REF	Green	Non-Unique Key Lookup	Low-medium -- Low if the number of matching rows is small; higher as the number of rows increases
FULLTEXT	Yellow	Fulltext Index Search	Specialized FULLTEXT search. Low -- for this specialized search requirement
REF_OR_NULL	Green	Key Lookup + Fetch NULL Values	Low-medium -- if the number of matching rows is small; higher as the number of rows increases
INDEX_MERGE	Green	Index Merge	Medium -- look for a better index selection in the query to improve performance
UNIQUE_SUBQUERY	Orange	Unique Key Lookup into table of subquery	Low -- Used for efficient Subquery processing
INDEX_SUBQUERY	Orange	Non-Unique Key Lookup into table of subquery	Low -- Used for efficient Subquery processing
RANGE	Orange	Index Range Scan	Medium -- partial index scan
INDEX	Red	Full Index Scan	High -- especially for large indexes
ALL	Red	Full Table Scan	Very High -- very costly for large tables, but less of an impact for small ones. No usable indexes were found for the table, which forces the optimizer to search every row. This could also mean that the search range is so broad that the index would be useless.
UNKNOWN	Black	unknown	Note: This is the default, in case a match cannot be determined

另外可参照https://dev.mysql.com/doc/refman/5.7/en/explain-output.html，用哪个版本的mysql就改成对应版本