Order of execution of join and correlated subquery works in SQL?-CodePudding

I have 3 tables, Table_A, Table_B, Table_C

Table_A has around 1000 rows,
Table_B has around 10 million rows,
Table_C has around 10 million rows,

Table_B and Table_C has around 7 million common rows

I joined all these three tables as per my requirement as below:

select *from Table_A A Left join Table_B B
on A.id=B.id
where
Not Exists
(select 1 from Table_C C where C.id=B.id )

So my Question is what will be the order of execution,i.e will the Table_B and Table_C be compared first on id and filter out common 7 million rows and then join will be executed with Table_A to filter 1000 rows which not exists in 7 million rows

Table_A will join with Table_B and fetch 1000 rows ,and then these 1000 rows of Table_B will be compared with Table_C

CodePudding user response：

The answer is entirely dependant on how your DBMS’s query optimiser decides to execute the query

CodePudding user response：

It depends by the DBMS or you could influencing the optimizer by using the optimizer hints.