On 07/09/15 15:37, Wei Zhang wrote:
Hi Andy,
I think there is criteria to choose better plan.
I will look at the source code. Hope it will help...
Thank you very much for your time.
Wei Zhang
Great - I look forward to hearing form you.
Andy
-----Original Message-----
From: Andy Seaborne [mailto:[email protected]]
Sent: Monday, 7 September 2015 11:56 PM
To: [email protected]
Subject: Re: Does TDB have command to see estimated query execution time and
row count ?
Hi there,
The optimizer does not try to estimate the execution time. It is not a fully
fledged, top-to-bottom cost-based optimizer. It does the best it can based on
heuristics. As has been discovered in Jena and elsewhere, SPARQL can also be
used in very simple fashion where the optimizer cost can be more than just
doing the query.
QueryExecUtils will actually execute the algebra expression. I mentioned the
class because you'll probably want to execute algebra directly to get
comparisons. And of course, you can look at the source code!
Andy
On 07/09/15 13:42, Wei Zhang wrote:
Hi Andy,
Thank you very much for your help!
I think per your suggestion, I can compare the performance with and without
optimizer.
But how can I get optimizer's estimated query time? Which I plan to compare
with the real execution time?
Do you mean when I execute algebra expressions directly using tools like
QueryExecUtils, then the time I get can be considered as estimated time?
I am not sure if my understanding is correct...
Best Regards,
Wei Zhang
-----Original Message-----
From: Andy Seaborne [mailto:[email protected]]
Sent: Monday, 7 September 2015 9:26 PM
To: [email protected]
Subject: Re: Does TDB have command to see estimated query execution time and
row count ?
On 07/09/15 05:30, Wei Zhang wrote:
Dear All,
From the document
(https://jena.apache.org/documentation/tdb/optimizer.html), it is said TDB
optimizer has both static and dynamic optimizations.
How can I get the estimated query time and row count instead of the actual time
and row count after static/dynamic optimization?
Another question is that I think "tdbquery -explain" gives the query plan after
execution, but it also cannot provide the information I want.
What I want is to find the TDB optimizer's performance.
Could anyone help?
Thank you very much for your time.
Best Regards,
Wei
Wei,
I think you have a model of how the optimizer works but it's at odds with what
it actually does. It is not strongly based around cost estimation although TDB
does a little of that.
The high level optimizations, done at the start of query execution, are a set
of rule based rewrites that look for patterns in the algebra and produce better
algebra. In particular, these are not based on the data.
Rather the rules are ways to standard SPARQL algebra (exactly as produced by the
transformation in the spec) into better (nearly always!) algebra. That includes
introducing a new operators (like "TopN") as well as rewriting using existing
operators (like filter/equality into a pattern with that term and a BIND).
This is printed by "qparse --print=opt"
TDB adds reordering basic graph patterns, either by the rule based method
described at that link or a fixed way (roughly - choose mist grounded triple
pattern, but avoid rdf:type).
There are tools (QueryExecUtils) to execute algebra expressions directly so
combined with the optimizer switched off, you can try out different
possibilities.
Andy
