select clause:

SELECTThe clause determines which column fields or expressions will be returned by a query statement

style.Unlike intuitive feelings, althoughSELECTLocatedSQLStart of statement, but

It is executed after the above clauses.After the other clauses are executed, SELECT

Applies the selected field or expression to each row of data.

Join performance:

In order to optimizeJOINQuery performance, firstShould follow the principle of big on the left and small on the right , which is

Place tables with small amounts of data on the right.This is because executingJOINWhen querying, regardless of the

Which connection method is used, the right table will all be loaded into memory for comparison with the left table.

Secondly,JOINQuery currently has no cache support , which means every timeJOINCheck

Query, even if the same is executed continuouslySQL, will also generate a new execution plan.

If the application will be heavily usedJOINInquiry, you need to further consider using the upper application

side caching service or useJOINTable engine to improve performance.

Finally,If it is a query scenario where a large number of dimension attributes are completed, it is recommended to use the word

Dictionary insteadJOINQuery .Because when a multi-table join query is performed, the query will be converted into two pairs

connected form, this“Snowball”-style queries are likely to cause performance problems.