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Avoid using a cursor in plpgsql's RETURN QUERY statement.

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plpgsql has always executed the query given in a RETURN QUERY command
by opening it as a cursor and then fetching a few rows at a time,
which it turns around and dumps into the function's result tuplestore.
The point of this was to keep from blowing out memory with an oversized
SPITupleTable result (note that while a tuplestore can spill tuples
to disk, SPITupleTable cannot).  However, it's rather inefficient, both
because of extra data copying and because of executor entry/exit
overhead.  In recent versions, a new performance problem has emerged:
use of a cursor prevents use of a parallel plan for the executed query.

We can improve matters by skipping use of a cursor and having the
executor push result tuples directly into the function's result
tuplestore.  However, a moderate amount of new infrastructure is needed
to make that idea work:

* We can use the existing tstoreReceiver.c DestReceiver code to funnel
executor output to the tuplestore, but it has to be extended to support
plpgsql's requirement for possibly applying a tuple conversion map.

* SPI needs to be extended to allow use of a caller-supplied
DestReceiver instead of its usual receiver that puts tuples into
a SPITupleTable.  Two new API calls are needed to handle both the
RETURN QUERY and RETURN QUERY EXECUTE cases.

I also felt that I didn't want these new API calls to use the legacy
method of specifying query parameter values with "char" null flags
(the old ' '/'n' convention); rather they should accept ParamListInfo
objects containing the parameter type and value info.  This required
a bit of additional new infrastructure since we didn't yet have any
parse analysis callback that would interpret $N parameter symbols
according to type data supplied in a ParamListInfo.  There seems to be
no harm in letting makeParamList install that callback by default,
rather than leaving a new ParamListInfo's parserSetup hook as NULL.
(Indeed, as of HEAD, I couldn't find anyplace that was using the
parserSetup field at all; plpgsql was using parserSetupArg for its
own purposes, but parserSetup seemed to be write-only.)

We can actually get plpgsql out of the business of using legacy null
flags altogether, and using ParamListInfo instead of its ad-hoc
PreparedParamsData structure; but this requires inventing one more
SPI API call that can replace SPI_cursor_open_with_args.  That seems
worth doing, though.

SPI_execute_with_args and SPI_cursor_open_with_args are now unused
anywhere in the core PG distribution.  Perhaps someday we could
deprecate/remove them.  But cleaning up the crufty bits of the SPI
API is a task for a different patch.

Per bug #16040 from Jeremy Smith.  This is unfortunately too invasive to
consider back-patching.  Patch by me; thanks to Hamid Akhtar for review.

Discussion: https://postgr.es/m/16040-eaacad11fecfb198@postgresql.org
This commit is contained in:
Tom Lane
2020-06-12 12:14:32 -04:00
parent aaf8c99050
commit 2f48ede080
9 changed files with 800 additions and 151 deletions
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63
src/backend/nodes/params.c
View File

@ -17,19 +17,27 @@
#include "access/xact.h"
#include "mb/stringinfo_mb.h"
#include "nodes/bitmapset.h"
#include "nodes/params.h"
#include "parser/parse_node.h"
#include "storage/shmem.h"
#include "utils/datum.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
static void paramlist_parser_setup(ParseState *pstate, void *arg);
static Node *paramlist_param_ref(ParseState *pstate, ParamRef *pref);
/*
* Allocate and initialize a new ParamListInfo structure.
*
* To make a new structure for the "dynamic" way (with hooks), pass 0 for
* numParams and set numParams manually.
*
* A default parserSetup function is supplied automatically. Callers may
* override it if they choose. (Note that most use-cases for ParamListInfos
* will never use the parserSetup function anyway.)
*/
ParamListInfo
makeParamList(int numParams)
@ -45,8 +53,8 @@ makeParamList(int numParams)
retval->paramFetchArg = NULL;
retval->paramCompile = NULL;
retval->paramCompileArg = NULL;
retval->parserSetup = NULL;
retval->parserSetupArg = NULL;
retval->parserSetup = paramlist_parser_setup;
retval->parserSetupArg = (void *) retval;
retval->paramValuesStr = NULL;
retval->numParams = numParams;
@ -102,6 +110,55 @@ copyParamList(ParamListInfo from)
return retval;
}
/*
* Set up to parse a query containing references to parameters
* sourced from a ParamListInfo.
*/
static void
paramlist_parser_setup(ParseState *pstate, void *arg)
{
pstate->p_paramref_hook = paramlist_param_ref;
/* no need to use p_coerce_param_hook */
pstate->p_ref_hook_state = arg;
}
/*
* Transform a ParamRef using parameter type data from a ParamListInfo.
*/
static Node *
paramlist_param_ref(ParseState *pstate, ParamRef *pref)
{
ParamListInfo paramLI = (ParamListInfo) pstate->p_ref_hook_state;
int paramno = pref->number;
ParamExternData *prm;
ParamExternData prmdata;
Param *param;
/* check parameter number is valid */
if (paramno <= 0 || paramno > paramLI->numParams)
return NULL;
/* give hook a chance in case parameter is dynamic */
if (paramLI->paramFetch != NULL)
prm = paramLI->paramFetch(paramLI, paramno, false, &prmdata);
else
prm = &paramLI->params[paramno - 1];
if (!OidIsValid(prm->ptype))
return NULL;
param = makeNode(Param);
param->paramkind = PARAM_EXTERN;
param->paramid = paramno;
param->paramtype = prm->ptype;
param->paramtypmod = -1;
param->paramcollid = get_typcollation(param->paramtype);
param->location = pref->location;
return (Node *) param;
}
/*
* Estimate the amount of space required to serialize a ParamListInfo.
*/