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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
Download Patch File Download Diff File Expand all files Collapse all files

159
src/backend/executor/spi.c
View File

@ -60,7 +60,8 @@ static void _SPI_prepare_oneshot_plan(const char *src, SPIPlanPtr plan);
static int _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
Snapshot snapshot, Snapshot crosscheck_snapshot,
bool read_only, bool fire_triggers, uint64 tcount);
bool read_only, bool fire_triggers, uint64 tcount,
DestReceiver *caller_dest);
static ParamListInfo _SPI_convert_params(int nargs, Oid *argtypes,
Datum *Values, const char *Nulls);
@ -513,7 +514,7 @@ SPI_execute(const char *src, bool read_only, long tcount)
res = _SPI_execute_plan(&plan, NULL,
InvalidSnapshot, InvalidSnapshot,
read_only, true, tcount);
read_only, true, tcount, NULL);
_SPI_end_call(true);
return res;
@ -547,7 +548,7 @@ SPI_execute_plan(SPIPlanPtr plan, Datum *Values, const char *Nulls,
_SPI_convert_params(plan->nargs, plan->argtypes,
Values, Nulls),
InvalidSnapshot, InvalidSnapshot,
read_only, true, tcount);
read_only, true, tcount, NULL);
_SPI_end_call(true);
return res;
@ -576,7 +577,36 @@ SPI_execute_plan_with_paramlist(SPIPlanPtr plan, ParamListInfo params,
res = _SPI_execute_plan(plan, params,
InvalidSnapshot, InvalidSnapshot,
read_only, true, tcount);
read_only, true, tcount, NULL);
_SPI_end_call(true);
return res;
}
/*
* Execute a previously prepared plan. If dest isn't NULL, we send result
* tuples to the caller-supplied DestReceiver rather than through the usual
* SPI output arrangements. If dest is NULL this is equivalent to
* SPI_execute_plan_with_paramlist.
*/
int
SPI_execute_plan_with_receiver(SPIPlanPtr plan,
ParamListInfo params,
bool read_only, long tcount,
DestReceiver *dest)
{
int res;
if (plan == NULL || plan->magic != _SPI_PLAN_MAGIC || tcount < 0)
return SPI_ERROR_ARGUMENT;
res = _SPI_begin_call(true);
if (res < 0)
return res;
res = _SPI_execute_plan(plan, params,
InvalidSnapshot, InvalidSnapshot,
read_only, true, tcount, dest);
_SPI_end_call(true);
return res;
@ -617,7 +647,7 @@ SPI_execute_snapshot(SPIPlanPtr plan,
_SPI_convert_params(plan->nargs, plan->argtypes,
Values, Nulls),
snapshot, crosscheck_snapshot,
read_only, fire_triggers, tcount);
read_only, fire_triggers, tcount, NULL);
_SPI_end_call(true);
return res;
@ -664,7 +694,50 @@ SPI_execute_with_args(const char *src,
res = _SPI_execute_plan(&plan, paramLI,
InvalidSnapshot, InvalidSnapshot,
read_only, true, tcount);
read_only, true, tcount, NULL);
_SPI_end_call(true);
return res;
}
/*
* SPI_execute_with_receiver -- plan and execute a query with arguments
*
* This is the same as SPI_execute_with_args except that parameters are
* supplied through a ParamListInfo, and (if dest isn't NULL) we send
* result tuples to the caller-supplied DestReceiver rather than through
* the usual SPI output arrangements.
*/
int
SPI_execute_with_receiver(const char *src,
ParamListInfo params,
bool read_only, long tcount,
DestReceiver *dest)
{
int res;
_SPI_plan plan;
if (src == NULL || tcount < 0)
return SPI_ERROR_ARGUMENT;
res = _SPI_begin_call(true);
if (res < 0)
return res;
memset(&plan, 0, sizeof(_SPI_plan));
plan.magic = _SPI_PLAN_MAGIC;
plan.cursor_options = CURSOR_OPT_PARALLEL_OK;
if (params)
{
plan.parserSetup = params->parserSetup;
plan.parserSetupArg = params->parserSetupArg;
}
_SPI_prepare_oneshot_plan(src, &plan);
res = _SPI_execute_plan(&plan, params,
InvalidSnapshot, InvalidSnapshot,
read_only, true, tcount, dest);
_SPI_end_call(true);
return res;
@ -1303,6 +1376,49 @@ SPI_cursor_open_with_paramlist(const char *name, SPIPlanPtr plan,
return SPI_cursor_open_internal(name, plan, params, read_only);
}
/*
* SPI_cursor_parse_open_with_paramlist()
*
* Same as SPI_cursor_open_with_args except that parameters (if any) are passed
* as a ParamListInfo, which supports dynamic parameter set determination
*/
Portal
SPI_cursor_parse_open_with_paramlist(const char *name,
const char *src,
ParamListInfo params,
bool read_only, int cursorOptions)
{
Portal result;
_SPI_plan plan;
if (src == NULL)
elog(ERROR, "SPI_cursor_parse_open_with_paramlist called with invalid arguments");
SPI_result = _SPI_begin_call(true);
if (SPI_result < 0)
elog(ERROR, "SPI_cursor_parse_open_with_paramlist called while not connected");
memset(&plan, 0, sizeof(_SPI_plan));
plan.magic = _SPI_PLAN_MAGIC;
plan.cursor_options = cursorOptions;
if (params)
{
plan.parserSetup = params->parserSetup;
plan.parserSetupArg = params->parserSetupArg;
}
_SPI_prepare_plan(src, &plan);
/* We needn't copy the plan; SPI_cursor_open_internal will do so */
result = SPI_cursor_open_internal(name, &plan, params, read_only);
/* And clean up */
_SPI_end_call(true);
return result;
}
/*
* SPI_cursor_open_internal()
@ -2090,11 +2206,13 @@ _SPI_prepare_oneshot_plan(const char *src, SPIPlanPtr plan)
* fire_triggers: true to fire AFTER triggers at end of query (normal case);
* false means any AFTER triggers are postponed to end of outer query
* tcount: execution tuple-count limit, or 0 for none
* caller_dest: DestReceiver to receive output, or NULL for normal SPI output
*/
static int
_SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
Snapshot snapshot, Snapshot crosscheck_snapshot,
bool read_only, bool fire_triggers, uint64 tcount)
bool read_only, bool fire_triggers, uint64 tcount,
DestReceiver *caller_dest)
{
int my_res = 0;
uint64 my_processed = 0;
@ -2228,6 +2346,12 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
bool canSetTag = stmt->canSetTag;
DestReceiver *dest;
/*
* Reset output state. (Note that if a non-SPI receiver is used,
* _SPI_current->processed will stay zero, and that's what we'll
* report to the caller. It's the receiver's job to count tuples
* in that case.)
*/
_SPI_current->processed = 0;
_SPI_current->tuptable = NULL;
@ -2267,7 +2391,16 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
UpdateActiveSnapshotCommandId();
}
dest = CreateDestReceiver(canSetTag ? DestSPI : DestNone);
/*
* Select appropriate tuple receiver. Output from non-canSetTag
* subqueries always goes to the bit bucket.
*/
if (!canSetTag)
dest = CreateDestReceiver(DestNone);
else if (caller_dest)
dest = caller_dest;
else
dest = CreateDestReceiver(DestSPI);
if (stmt->utilityStmt == NULL)
{
@ -2373,7 +2506,13 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
SPI_freetuptable(_SPI_current->tuptable);
_SPI_current->tuptable = NULL;
}
/* we know that the receiver doesn't need a destroy call */
/*
* We don't issue a destroy call to the receiver. The SPI and
* None receivers would ignore it anyway, while if the caller
* supplied a receiver, it's not our job to destroy it.
*/
if (res < 0)
{
my_res = res;
@ -2465,7 +2604,7 @@ _SPI_pquery(QueryDesc *queryDesc, bool fire_triggers, uint64 tcount)
switch (operation)
{
case CMD_SELECT:
if (queryDesc->dest->mydest != DestSPI)
if (queryDesc->dest->mydest == DestNone)
{
/* Don't return SPI_OK_SELECT if we're discarding result */
res = SPI_OK_UTILITY;