Re: [sqlite] beginner question

[Qiang]

--- 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


 

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[sqlite] beginner question

[Qiang]

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?

thanks,

Qiang


 

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[sqlite] Question about sqlite TEMP table

[Ming Qiang Lin]

Hi All,

Following is the sqlite CREATE TABLE definition:

If the "TEMP" or "TEMPORARY" keyword occurs in between "CREATE" and
"TABLE" then the table that is created is only visible to the process
that opened the database and is automatically deleted when the
database is closed.  Any indices created on a temporary table are also
temporary.  Temporary tables and indices are stored in a separate file
distinct from the main database file.

 If a  is specified, then the table is created in  the
named database. It is an error to specify both a  and
the TEMP keyword, unless the  is "temp". If no database
name is specified, and the TEMP keyword is not present, the table is
created in the main database.


It says that temp table is visible to the process it created, but how
about in this same process,  I join a temp table with non-temp table?.
Since I am having Exception says: "no such table" when I do the join.

Please help!

Thank you in advance.


Mica