How to look at the Expression Trees

On 21.03.2011 13:44, Vaibhav Kaushal wrote:

Hi all,

I have been trying to work on the expression evaluator (trying to alter
it just for the seqscan case). I have understood a few things. I wish
someone could tell me if I am wrong at some point. As far as I have gone
through the code, I think:

1. Quals are formulated by planner
2. Quals are actually a list of Expression Trees.
3. They are created in the ExecInitExpr function.
4. Every row / tuple table slot is run through the same qual list and
thus goes through the same expression tree execution path as the other
(of course being filtered in between if they do not fit the qual in the
list).

Yes. There's actually two "trees" involved. The planner produces a tree
of Expr nodes, and ExecInitExpr prepares a tree of ExprState nodes that
mirrors the first tree. The ExprStates contain run-time state needed to
execute the Expr tree.

5. The most common nodes in the expression trees are the ExecEvalVar,
ExecEvalConst and ExecEvalParam.

Well, that's obviously going to depend on the query.

I might be wrong somewhere (especially most people would be able to say
a lot about the 5th point). But if the above were to be correct then how
and why are the ExecMakeFunctionResultNoSets, ExecEvalRelabelType,
ExecEvalFuncArgs and the likes are used?

ExecMakeFunctionResultNoSets is used to evaluate function calls. The
first function call invocation always uses ExecMakeFunctionResult, but
if ExecMakeFunctionResult sees on that first invocation that it was not
a set-returning-function, it changes the evaluator function for
subsequent invocations to ExecMakeFunctionResultNoSets.
ExecMakeFunctionResultNoSets does the same thing as
ExecMakeFunctionResult, but skips the checks for set-returning
functions, making the evaluation a bit faster.

ExecEvalFuncArgs is used by ExecMakeFunctionResult to evaluate the
function arguments.

ExecEvalRelabelType is used to evaluate RelabelType nodes. RelabelType
doesn't really do anything, it's just a placeholder when a type is cast
to another, and the source and target types are binary compatible.

I wanted to see how the expression tree gets into form before it gets
into the ExecQual for parse by ExecEvalExpr function. Is there a way to
see the Expression Tree so that I get a better idea about what is
happening?

set debug_print_plan=on

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

On Mon, Mar 21, 2011 at 5:47 PM, Heikki Linnakangas <
heikki.linnakangas@enterprisedb.com> wrote:

Yes. There's actually two "trees" involved. The planner produces a tree of
Expr nodes, and ExecInitExpr prepares a tree of ExprState nodes that mirrors
the first tree. The ExprStates contain run-time state needed to execute the
Expr tree.

Yes I am seeing that in the code.

5. The most common nodes in the expression trees are the ExecEvalVar,

ExecEvalConst and ExecEvalParam.

Well, that's obviously going to depend on the query.

Indeed yes, it does, but still for most cases the ExecEvalVar or

ExecEvalScalarVar is seen a lot of times (I am trying only simple queries
which take most data from disk) which is why i said that. Anyways you are
right there.

I might be wrong somewhere (especially most people would be able to say

a lot about the 5th point). But if the above were to be correct then how
and why are the ExecMakeFunctionResultNoSets, ExecEvalRelabelType,
ExecEvalFuncArgs and the likes are used?

ExecMakeFunctionResultNoSets is used to evaluate function calls. The first
function call invocation always uses ExecMakeFunctionResult, but if
ExecMakeFunctionResult sees on that first invocation that it was not a
set-returning-function, it changes the evaluator function for subsequent
invocations to ExecMakeFunctionResultNoSets. ExecMakeFunctionResultNoSets
does the same thing as ExecMakeFunctionResult, but skips the checks for
set-returning functions, making the evaluation a bit faster.

ExecEvalFuncArgs is used by ExecMakeFunctionResult to evaluate the function
arguments.

ExecEvalRelabelType is used to evaluate RelabelType nodes. RelabelType
doesn't really do anything, it's just a placeholder when a type is cast to
another, and the source and target types are binary compatible.

did not know the last fact.

I wanted to see how the expression tree gets into form before it gets

into the ExecQual for parse by ExecEvalExpr function. Is there a way to
see the Expression Tree so that I get a better idea about what is
happening?

set debug_print_plan=on

I am already using the postgresql server with -d 4 option and it shows a

lot of things. But I am not able to see the Expression State trees. OK. I
know that the output DOES show the 'expr' entries. But if those are what
make the Expression _tree_ then I am not able to understand them. A little
help on that would be generous.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

Regards,
Vaibhav

On 21.03.2011 14:37, Vaibhav Kaushal wrote:

I am already using the postgresql server with -d 4 option and it shows a
lot of things. But I am not able to see the Expression State trees.

To be precise, debug_print_plan=on prints the expression tree that comes
from planner, not the execution tree of ExprStates. But the structure of
the ExprState tree is the same as the planner tree, you just have
ExprState nodes in place of Expr nodes.

OK. I
know that the output DOES show the 'expr' entries. But if those are what
make the Expression _tree_ then I am not able to understand them. A little
help on that would be generous.

It is indeed a tree that gets printed with debug_print_plan. There's
more than the expression tree there, with information about planner the
relations involved etc. Look for "targetlist" and "qual" fields in the
output, that's where the expression trees are.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

Hi,

You said:

ExecMakeFunctionResultNoSets is used to evaluate function calls.

What are the 'functions' there? Are they the user supplied pl/PGSQL style
user functions, the functions handled by fmgr or are they just another C
function which make the Expression Evaluator?

Regards,
Vaibhav

On 22.03.2011 05:39, Vaibhav Kaushal wrote:

You said:

ExecMakeFunctionResultNoSets is used to evaluate function calls.

What are the 'functions' there? Are they the user supplied pl/PGSQL style
user functions, the functions handled by fmgr or are they just another C
function which make the Expression Evaluator?

Functions you can use in queries, written in PL/pgSQL or other PL
languages. Including user-defined and built-in functions, and functions
implementing operators.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com