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[IndexedDB] Multi-value keys

From: Jonas Sicking <jonas@sicking.cc>
Date: Fri, 18 Jun 2010 16:08:12 -0700
Message-ID: <AANLkTin48SD3Fo1ajhDNSSymYQDGFYOusfXYr9JMPqoR@mail.gmail.com>
To: Webapps WG <public-webapps@w3.org>
Hi All,

One thing that (if I'm reading the spec correctly) is currently
impossible is to create multi-valued keys. Consider for example an
object store containing objects like:

{ firstName: "Sven", lastName: "Svensson", age: 57 }
{ firstName: "Benny", lastName: "Andersson", age: 63 }
{ firstName: "Benny", lastName: "Bedrup", age: 9 }

It is easy to create an index which lets you quickly find everyone
with a given firstName or a given lastName. However it doesn't seem
possible to create an index that finds everyone with a given firstName
*and* lastName, or sort the list of people based on firstName and then
lastName.

The best thing you could do is to concatenate the firstname and
lastname and insert a ascii-null character in between and then use
that as a key in the index. However this doesn't work if firstName or
lastName can contain null characters. Also, if you want to be able to
sort by firstName and then age there is no good way to put all the
information into a single string while having sorting work.

Generally the way this is done in SQL is that you can create an index
on multiple columns. That way each row has multiple values as the key,
and sorting is first done on the first value, then the second, then
the third etc.

However since we don't really have columns we can't use that exact
solution. Instead, the way we could allow multiple values is to add an
additional type as keys: Arrays.

That way you can use ["Sven",  57], ["Benny", 63] and ["Benny", 9] as
keys for the respective objects above. This would allow sorting and
searching on firstName and age.

The way that array keys would be compared is that we'd first compare
the first item in both arrays. If they are different the arrays are
ordered the same way as the two first-values are order. If they are
the same you look at the second value and so on. If you reach the end
of one array before finding a difference then that array is sorted
before the other.

We'd also have to define the order if an array is compared to a
non-array value. It doesn't really matter what we say here, but I
propose that we put all array after all non-arrays.

Note that I don't think we need to allow arrays to contain arrays.
That just seems to add complication without adding additional
functionality.

Let me know what you think.

/ Jonas
Received on Friday, 18 June 2010 23:09:04 GMT

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