PL/Python prepare example's use of setdefault

On 10/15/2014 02:39 PM, Jonathan Rogers wrote:

I was just reading the PL/Python docs section "42.7.1 Database Access
Functions" and saw this example:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
plan = SD.setdefault("plan", plpy.prepare("SELECT 1"))
# rest of function
$$ LANGUAGE plpythonu;

The above example uses the plpy.prepare() function, reusing the result
across function calls uses setdefault(). Unfortunately, since
setdefault() is a method on dict objects, the values passed to it must
be evaluated before it can be called. Therefore, plpy.prepare() will be
called every time usesavedplan() executes whether a result already
exists in the SD dict or not.

I'm not sure if it's a problem that plpy.prepare() is called every time
since the result is discarded if a prepared statement had been cached by
a previous execution of usesavedplan(). It seems that some wasted
processing will occur, but maybe not enough to matter. The documentation
for SPI_prepare() does not clearly state what tasks that function
performs other than constructing a prepared statement object. It seems
to imply that parsing does occur within SPI_prepare(). It does state
that query planning occurs within SPI_execute_plan().

Can anyone clarify what occurs when plpy.prepare() is called? Is it
worth using a Python conditional to determine whether to call it rather
than using SD.setdefault()?

Like in the older documentation?:

http://www.postgresql.org/docs/9.1/static/plpython-database.html

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
if SD.has_key("plan"):
plan = SD["plan"]
else:
plan = plpy.prepare("SELECT 1")
SD["plan"] = plan
# rest of function
$$ LANGUAGE plpythonu;

--
Adrian Klaver
adrian.klaver@aklaver.com

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Adrian Klaver <adrian.klaver@aklaver.com> writes:

On 10/15/2014 02:39 PM, Jonathan Rogers wrote:

I was just reading the PL/Python docs section "42.7.1 Database Access
Functions" and saw this example:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
plan = SD.setdefault("plan", plpy.prepare("SELECT 1"))
# rest of function
$$ LANGUAGE plpythonu;

The above example uses the plpy.prepare() function, reusing the result
across function calls uses setdefault(). Unfortunately, since
setdefault() is a method on dict objects, the values passed to it must
be evaluated before it can be called. Therefore, plpy.prepare() will be
called every time usesavedplan() executes whether a result already
exists in the SD dict or not.

Can anyone clarify what occurs when plpy.prepare() is called? Is it
worth using a Python conditional to determine whether to call it rather
than using SD.setdefault()?

Like in the older documentation?:

Hm ... this was changed in commit 6f6b46c9c0ca3d96. Peter, did
you consider efficiency here?

regards, tom lane

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On 10/15/2014 05:51 PM, Adrian Klaver wrote:

On 10/15/2014 02:39 PM, Jonathan Rogers wrote:

I was just reading the PL/Python docs section "42.7.1 Database Access
Functions" and saw this example:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
plan = SD.setdefault("plan", plpy.prepare("SELECT 1"))
# rest of function
$$ LANGUAGE plpythonu;

The above example uses the plpy.prepare() function, reusing the result
across function calls uses setdefault(). Unfortunately, since
setdefault() is a method on dict objects, the values passed to it must
be evaluated before it can be called. Therefore, plpy.prepare() will be
called every time usesavedplan() executes whether a result already
exists in the SD dict or not.

I'm not sure if it's a problem that plpy.prepare() is called every time
since the result is discarded if a prepared statement had been cached by
a previous execution of usesavedplan(). It seems that some wasted
processing will occur, but maybe not enough to matter. The documentation
for SPI_prepare() does not clearly state what tasks that function
performs other than constructing a prepared statement object. It seems
to imply that parsing does occur within SPI_prepare(). It does state
that query planning occurs within SPI_execute_plan().

Can anyone clarify what occurs when plpy.prepare() is called? Is it
worth using a Python conditional to determine whether to call it rather
than using SD.setdefault()?

Like in the older documentation?:

http://www.postgresql.org/docs/9.1/static/plpython-database.html

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
if SD.has_key("plan"):
plan = SD["plan"]
else:
plan = plpy.prepare("SELECT 1")
SD["plan"] = plan
# rest of function
$$ LANGUAGE plpythonu;

Exactly. It seems to me that the approach taken by the newer
documentation will be less efficient. If so, why was the example
changed? BTW, I would rewrite the 9.1 example to be shorter while
behaving the same:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
plan = SD.get("plan")
if plan is None:
SD["plan"] = plan = plpy.prepare("SELECT 1")
# rest of function
$$ LANGUAGE plpythonu;

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Jonathan Ross Rogers

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On 10/15/14 5:56 PM, Tom Lane wrote:

Hm ... this was changed in commit 6f6b46c9c0ca3d96. Peter, did
you consider efficiency here?

Fixed.

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On 10/15/14 5:58 PM, Jonathan Rogers wrote:

BTW, I would rewrite the 9.1 example to be shorter while
behaving the same:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
plan = SD.get("plan")
if plan is None:

If we're going for shortness, how about

if not plan:

?

SD["plan"] = plan = plpy.prepare("SELECT 1")

and here maybe

plan = SD["plan"] = plpy.prepare("SELECT 1")

to emphasize the assignment to "plan"?

# rest of function
$$ LANGUAGE plpythonu;

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On 11/01/2014 12:13 PM, Peter Eisentraut wrote:

On 10/15/14 5:58 PM, Jonathan Rogers wrote:

BTW, I would rewrite the 9.1 example to be shorter while
behaving the same:

CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
plan = SD.get("plan")
if plan is None:

If we're going for shortness, how about

if not plan:

Sure, that's fine as long as a plan object never looks Falsey.

?

SD["plan"] = plan = plpy.prepare("SELECT 1")

and here maybe

plan = SD["plan"] = plpy.prepare("SELECT 1")

to emphasize the assignment to "plan"?

Yeah, order of assignment shouldn't matter.

# rest of function
$$ LANGUAGE plpythonu;

--
Jonathan Ross Rogers

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