tsql - T-Sql Algorithm Question -

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i have t-sql statement follows;

insert table1 select * table2

i want know running sequence. insert waits select statement finish before starting or starts asap select statement starts returning values , expects new records select statement continue.

this plain stored procedure , no transactions used.

to echo @codebymoonlight's answer, , address comment there: physical considerations (including specifics of locking) subordinate logical instructions specified query.

in processing of insert ... select statement, logically speaking select carried out produce resultset, , rows of resultset inserted. fact in case source table , target table same table irrelevant. i'm sure specifying nolock or tablock in case apply select, if that's position them.

consider example statement, makes no sense if read in 'imperative' way:

update sometable set column1 = column2, column2 = column1

with imperative, rather set-based, understanding, statement might if result in column1 , column2 having same value rows. doesn't - in fact swaps values in column1 , column2. understanding logical instructions of query dictate happens can seen.


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