Hi, guys, I recently learning SQL optimization, hope to get a don't understand below the bosses give directions, thank you very much,
Article 1: for id=4 query returns the number of 10 but according to this article 10 rowid to query id=3 why actual only eight records?
2: in the cycle of EMP as driver table EMP actual only have 10 records have been found out why cycle only eight records DEPT will query the 23 records?
The above two questions hope to help answer the great god, thank you very much,

CodePudding user response:

1, the rows in the execution plan is valuation
2, the join link way is matching field. Descartes product, return to record more than a table is normal

CodePudding user response:

Implementation plan of valuation and the actual results, there are some differences,


1: for id=4 query returns a number is 10 but according to this article 10 rowid to query id=3 why actual only eight records?


Just stick data are up to,

CodePudding user response:

No data, id=3 time to start is a - 10 data rows are not carry out the number of rows returned