<quote>simply because a row can't contain sub-arrays or hashes</quote>
This is not true at all. At least in PostgreSQL you can have arrays and hashes:
This is not true at all. At least in PostgreSQL you can have arrays and hashes:
"Tip: Arrays are not sets; searching for specific array elements can be a sign of database misdesign. Consider using a separate table with a row for each item that would be an array element. This will be easier to search, and is likely to scale better for a large number of elements."
Searching an array is a pretty common task, Mongo does really well in its ability to search into objects in a document.
I had to laugh a little at one of the sample queries:
SELECT f1[1][-2][3] AS e1, f1[1][-1][5] AS e2 FROM (SELECT '[1:1][-2:-1][3:5]={{{1,2,3},{4,5,6}}}'::int[] AS f1) AS ss;
Seriously?
About the performance/scalability warning; I don't deal with very large arrays, a couple hundred items max, and when using a GIN index over the array field, search queries are screamingly fast.
http://www.mongodb.org/display/DOCS/Dot+Notation+%28Reaching...
From the "Array Element by Position" example:
db.blogposts.find( { "comments.0.by" : "Abe" } )
If you stored XML documents in PostgreSQL the similar query to the MongoDB one would be something like this:
select * from blogposts where (xpath('/comments[0]/@by', doc))::text[] = array['Abe']