Re: Native XML - Mailing list pgsql-hackers

On 03/09/2011 08:21 PM, Yeb Havinga wrote: <blockquote cite="mid:4D77D31F.9060501@gmail.com" type="cite"></blockquote>
On2011-03-09 19:30, Robert Haas wrote: <blockquote
cite="mid:AANLkTi=E+Lamz7onQ_w1uS55a5ymGjpWMqrv8eDH1Cmb@mail.gmail.com"type="cite"><pre wrap="">On Wed, Mar 9, 2011 at
1:11PM, Bruce Momjian <a class="moz-txt-link-rfc2396E" href="mailto:bruce@momjian.us"
moz-do-not-send="true"><bruce@momjian.us></a>wrote:   </pre><blockquote type="cite"><pre wrap="">Robert Haas
wrote:    </pre><blockquote type="cite"><pre wrap="">On Mon, Feb 28, 2011 at 10:30 AM, Tom Lane <a
class="moz-txt-link-rfc2396E"href="mailto:tgl@sss.pgh.pa.us" moz-do-not-send="true"><tgl@sss.pgh.pa.us></a>
wrote:      </pre><blockquote type="cite"><pre wrap="">Well, in principle we could allow them to work on both, just the
same
way that (for instance) "+" is a standardized operator but works on more
than one datatype. ?But I agree that the prospect of two parallel types
with essentially duplicate functionality isn't pleasing at all.         </pre></blockquote><pre wrap="">The real issue
hereis whether we want to store XML as text (as we do
 
now) or as some predigested form which would make "output the whole
thing" slower but speed up things like xpath lookups.  We had the same
issue with JSON, and due to the uncertainty about which way to go with
it we ended up integrating nothing into core at all.  It's really not
clear that there is one way of doing this that is right for all use
cases.  If you are storing xml in an xml column just to get it
validated, and doing no processing in the DB, then you'd probably
prefer our current representation.  If you want to build functional
indexes on xpath expressions, and then run queries that extract data
using other xpath expressions, you would probably prefer the other
representation.       </pre></blockquote><pre wrap="">Someone should measure how much overhead the indexing of xml
values
might have.  If it is minor, we might be OK with only an indexed xml
type.     </pre></blockquote><pre wrap="">I think the relevant thing to measure would be how fast the
predigested representation speeds up the evaluation of xpath
expressions.   </pre></blockquote> About a predigested representation, I hope I'm not insulting anyone's education
here,but a lot of XML database 'accellerators' seem to be using the pre and post orders (see <a
class="moz-txt-link-freetext"href="http://en.wikipedia.org/wiki/Tree_traversal"
moz-do-not-send="true">http://en.wikipedia.org/wiki/Tree_traversal</a>)of the document nodes. The following two pdfs
showhow these orders can be used to query for e.g. all ancestors of a node: second pdf slide 10: for nodes x,y : x is
anancestor of y when x.pre < y.pre AND x.post > y.post.<br /><br /><a class="moz-txt-link-abbreviated"
href="http://www.cse.unsw.edu.au/%7Ecs4317/09s1/tutorials/tutor4.pdf"
moz-do-not-send="true">www.cse.unsw.edu.au/~cs4317/09s1/tutorials/tutor4.pdf</a> about the format<br /><a
class="moz-txt-link-abbreviated"href="http://www.cse.unsw.edu.au/%7Ecs4317/09s1/tutorials/tutor10.pdf"
moz-do-not-send="true">www.cse.unsw.edu.au/~cs4317/09s1/tutorials/tutor10.pdf</a>about querying the format<br /><br />
regards,<br/> Yeb Havinga<span id="search"><span class="f"><cite><br /></cite></span></span><br />  This looks rather
likea special kind of XML shredding and that is listed at <a class="moz-txt-link-freetext"
href="http://wiki.postgresql.org/wiki/Todo">http://wiki.postgresql.org/wiki/Todo</a><br/><br /> About the predigested /
plainstorage and the evaluation: I haven't yet fully given up the idea to play with it, even though on purely
experimentalbasis (i.e. with little or no ambition to contribute to the core product). If doing so, interesting might
alsobe to use TOAST slicing during the xpath evaluation so that large documents are not fetched immediately as a whole,
ifthe xpath is rather 'short'.<br /><br /> But first I should let all the thoughts 'settle down'. There may well be
otherareas of Postgres where it's worth to spend some time, whether writing something or just reading.<br />