Re: Re: how to find the order of joins from Explain command XML plan output in PostgreSQL

<p dir="ltr">Le 9 juil. 2014 20:36, "David G Johnston" <<a
href="mailto:david.g.johnston@gmail.com">david.g.johnston@gmail.com</a>>a écrit :<br /> ><br /> > csrajmohan
wrote<br/> > > "EXPLAIN (format XML) " command in PostgreSQL9.3.4 gives the plan chosen<br /> > > by<br />
>> the optimizer in XML format. In my program, I have to extract certain data<br /> > > about optimizer
planfrom this XML output. I am using *LibXML2* library<br /> > > for<br /> > > parsing the XML. I had
successfullyextracted information about which<br /> > > relations are involved and what joins are used by parsing
theXML. But<br /> > > I am *unable<br /> > > to extract the* *order of joining the relations from the XML
output*.I<br /> > > conceptually understood that the reverse level order traversal of binary<br /> > > tree
representationof the XML plan will give correct ordering of joins<br /> > > applied. But I could not figure out
howdo I get that from the XML? Does<br /> > > libXML2 support anything of this sort? If not how should I proceed
to<br/> > > tackle this?<br /> ><br /> > So, since nothing better has been forthcoming in your other two
postson<br /> > this topic I'll just say that likely you will have much better luck using<br /> > SAX-based
processingas opposed to DOM-based processing.  I seriously doubt<br /> > native/core PostgreSQL facilities will
allowyou to do what you desire.<br /> ><br /> > As you said, hierarchy and physical output order determines the
"orderof<br /> > joining" within the planner so you have to capture and track such relational<br /> > information
duringyour processing - which is made much easier if you simply<br /> > traverse the output node-by-node exactly as
aSAX based parser does.<br /> ><br /> > Though pgAdminIII has a visual query display that you might look at
for<br/> > inspiration.<br /> ><p dir="ltr">FWIW, pgadmin's visual explain doesn't (yet?) use XML or json or yaml
output.