Redundancy of multifield indexes?

[Bruce Rusk]

I have a question about indexing multiple fields and whether it's redundant
to index the individual fields separately.

As I understand it, if there is an index on two fields, say idxFullName
which includes LastName and FirstName, in that order, it would be redundant
to create a separate index on LastName since idxFullName would already be
indexing LastName. But it would be necessary to create a separate index for
FirstName to assist with searches on FirstName alone (e.g. for all "Bob"s or
"Mary"s regardless of surname).

My question is: does this apply to indexes that include three or more
fields?

For example, if I have a table with LastName, FirstName and MiddleName and
create an index on all three fields (in that order), and then do a search on
LastName = x and FirstName = y, will the Jet Engine be able to figure out
that it can use the first two fields of the index? Similarly, will this
three-field index also be helpful for searches covering just the LastName
field?

If anyone knows the answer (or can point me to someplace in the Access/Jet
documentation that I have overlooked), thanks in advance.

Bruce Rusk

[Barry Edmund Wright]

Hi Bruce,

I have a tblNames that has strFName strLName fields. I use four indexes on
this table:
LName = strLName
FName = strFName
LastName = strLName and strFName
FirstName = strFName and strLName

I search on both the first and last names or only on the first name or only
on the last name. The database has 10,000 names and the records are returned
instantly (PIII 600).

I do not specify an index and have even found that doing so can actually
slow the searches in general down. Which makes me think that Jet4 is smarter
than I am when it comes to selecting the best index.

Experience is the best teacher, just do a little experimenting.
Regards,
Barry

"Bruce Rusk" <br************ ***@stanford.ed oo> wrote in message
news:Pa******** ************@co mcast.com...

I have a question about indexing multiple fields and whether it's redundant
to index the individual fields separately.

As I understand it, if there is an index on two fields, say idxFullName
which includes LastName and FirstName, in that order, it would be
redundant
to create a separate index on LastName since idxFullName would already be
indexing LastName. But it would be necessary to create a separate index
for
FirstName to assist with searches on FirstName alone (e.g. for all "Bob"s
or
"Mary"s regardless of surname).

My question is: does this apply to indexes that include three or more
fields?

For example, if I have a table with LastName, FirstName and MiddleName and
create an index on all three fields (in that order), and then do a search
on
LastName = x and FirstName = y, will the Jet Engine be able to figure out
that it can use the first two fields of the index? Similarly, will this
three-field index also be helpful for searches covering just the LastName
field?

If anyone knows the answer (or can point me to someplace in the Access/Jet
documentation that I have overlooked), thanks in advance.

Bruce Rusk

[Allen Browne]

Answered in microsoft.publi c.access.tabled esign

[David W. Fenton]

"Allen Browne" <Al*********@Se eSig.Invalid> wrote in
news:42******** *************** @per-qv1-newsreader-01.iinet.net.au :

Answered in microsoft.publi c.access.tabled esign

Why? The post was only posted here in CDMA.

I'd be interested to know how you answered it, since I'm pretty sure
that the number of indexes you need is N-1, where N is the number of
columns in the multi-column index.

--
David W. Fenton http://www.bway.net/~dfenton
dfenton at bway dot net http://www.bway.net/~dfassoc

[Steve Jorgensen]

On Wed, 6 Jul 2005 16:00:49 -0700, "Bruce Rusk"
<br************ ***@stanford.ed oo> wrote:

I have a question about indexing multiple fields and whether it's redundant
to index the individual fields separately.

As I understand it, if there is an index on two fields, say idxFullName
which includes LastName and FirstName, in that order, it would be redundant
to create a separate index on LastName since idxFullName would already be
indexing LastName. But it would be necessary to create a separate index for
FirstName to assist with searches on FirstName alone (e.g. for all "Bob"s or
"Mary"s regardless of surname).

My question is: does this apply to indexes that include three or more
fields?

For example, if I have a table with LastName, FirstName and MiddleName and
create an index on all three fields (in that order), and then do a search on
LastName = x and FirstName = y, will the Jet Engine be able to figure out
that it can use the first two fields of the index? Similarly, will this
three-field index also be helpful for searches covering just the LastName
field?

If anyone knows the answer (or can point me to someplace in the Access/Jet
documentatio n that I have overlooked), thanks in advance.

Bruce Rusk

If I have, say, a 3-field identifier of some kind, I'll try to make an index
for the combination, and I'll make indexes for any individual fields I think
are highly likely to be searched or sorted individually. I'll make the first
field of the multi-field index -not- be one of those because then the
multi-field index can at leas be some help in the less common case, though
it's bigger and therefore slower to search than the single-field indexes. Be
careful not to go overboard because more indexes adds to the time it takes to
do updates and increases the likelihood of multi-user lock conflicts. For
fields less commonly searched on, it may be better to use no index and live
with the table scan penalty for those.

I have some uncertainty about how clever Access can get in using indexes
because I know that SQL Server can be really damn clever. For instance, if
I'm searching on the second field in a multi-field index, SQL Server may still
use the index even though it has to do a full scan of it because the estimated
number of reads is still smaller that way than doing the full scan of the
table first. I don't know if Access/Jet does that kind of optimization or
not.

[Allen Browne]

Bruce posted the same question under a different topic in the ms newsgroup.

The reply on server news.microsoft. com was:
news:e4******** ******@TK2MSFTN GP12.phx.gbl

--
Allen Browne - Microsoft MVP. Perth, Western Australia.
Tips for Access users - http://allenbrowne.com/tips.html
Reply to group, rather than allenbrowne at mvps dot org.

"David W. Fenton" <dX********@bwa y.net.invalid> wrote in message
news:Xn******** *************** *********@24.16 8.128.90...

"Allen Browne" <Al*********@Se eSig.Invalid> wrote in
news:42******** *************** @per-qv1-newsreader-01.iinet.net.au :

Answered in microsoft.publi c.access.tabled esign

Why? The post was only posted here in CDMA.

I'd be interested to know how you answered it, since I'm pretty sure
that the number of indexes you need is N-1, where N is the number of
columns in the multi-column index.