--- Dennis Cote <[EMAIL PROTECTED]> wrote: > Qiang wrote: > > hello, > > > > two questions.. > > > > 1. comparing select one or two columns from one > row > > from a bigger table ( 20,000 - 30,000 rows, 20 > columns > > ). > > with > > select the same column from one row from a > smaller > > table (20,000 - 30,000 rows, 2 columns). does it > make > > much different on performance? if yes, what makes > it? > > > > 2. Comparing select a sequence number first from a > > small table THEN select all rows from another > table > > that have the same sequence number ( hence two > selects > > ) > > with > > join select on this two tables where they have > the > > same sequence number. any difference here? > > > > > Qiang, > > Re 1: It will be slightly faster to get the data > from a table with fewer > columns if the required columns are at the end of > the row since sqlite > has to skip over the unused fields. But unless you > have large fields the > difference will be very small. It is more important > to reduce the number > of rows to scan, or provide an index so sqlite can > locate the correct > row more quickly. > > Re 2: I suspect the join will always be faster since > it essentially does > the same thing but without the overhead of executing > a second statement. > If you only return the columns from the second table > it will give the > same results. >
thank you for the answer. the reason i asked because i want to combine few smaller table into one and save the time join all of them. your answer helped me see the the consequence. > HTH > Dennis Cote > Qiang ____________________________________________________________________________________ Sponsored Link $200,000 mortgage for $660/ mo - 30/15 yr fixed, reduce debt - http://yahoo.ratemarketplace.com ----------------------------------------------------------------------------- To unsubscribe, send email to [EMAIL PROTECTED] -----------------------------------------------------------------------------
