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SPI programming guide.

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Vadim B. Mikheev 1997-09-29 08:29:09 +00:00
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PostgreSQL Server Programming Interface
Server Programming Interface (SPI) is attempt to give users ability run
SQL-queries inside user-defined C-function. For lack of Procedural Language
(PL) in current version of PostgreSQL, SPI is only way to write server
stored procedures and triggers. In the future, SPI will be used as
"workhorse" for PL.
Actually, SPI is just set of builtin interface functions to simplify
access to Parser/Planner/Optimizer and Executor. Also, SPI does some memory
management.
To avoid misunderstanding we'll use word "function" for SPI interface
functions and word "procedure" for user-defined C-functions using SPI.
SPI procedures are always called by some (upper) Executor and SPI manager
uses Executor to run your queries. Other procedures may be called by
Executor running queries from your procedure.
Note, that if during execution of query from a procedure transaction will
be aborted then control will not be returned to your procedure - all work
will be rollbacked and server will wait for the next command from client.
It will be changed in the next versions.
Other restrictions are unability to execute BEGIN, END and ABORT
(transaction control statements) and cursor operations.
These are also to be changed in future.
Interface functions
If successful, SPI functions returns non-negative result (either via
returned (int) value or in SPI_result global variable, as described below).
Otherwise, negative result will be returned.
int SPI_connect (void)
Connects your procedure to SPI manager. Initializes SPI internal
structures for query execution and memory management.
You are to call this function if you need in execution of queries. Some
utility SPI functions may be called from un-connected procedures.
Returns:
SPI_OK_CONNECT if connected.
SPI_ERROR_CONNECT if not. You may get this error if SPI_connect() is
called from already connected procedure - e.g. if you directly call one
procedure from another connected one. Actually, while child procedure
will be able to use SPI, your parent procedure will not be able continue
use SPI after child returned (if SPI_finish() called by child). It's bad
practice.
int SPI_finish(void)
Dis-connects your procedure from SPI manager. Frees all memory
allocations made by your procedure via palloc() after SPI_connect().
These allocations can't be used any more! See Memory management.
After SPI_finish() is called your procedure loses ability to run queries.
Server is in the same state as just before call to SPI_connect().
Returns:
SPI_OK_FINISH if properly disconnected.
SPI_ERROR_UNCONNECTED if called from un-connected procedure. No problems
with this - it means that nothing was made by SPI manager.
NOTE! SPI_finish() MUST be called by connected procedure or you may get
unpredictable results!
int SPI_exec(char *query, int tcount)
Creates execution plan (parser+planner+optimizer) and executes query for
tcount tuples. Should be called from connected procedure. If tcount eq 0
then executes query for all tuples returned by query scan. Using tcount >
0 you may restrict number of tuples for which query will be executed:
SPI_exec ("insert into _table_ select * from _table_", 5);
- at max 5 tuples will be inserted into _table_.
If execution of your query was successful then one of the next
(non-negative) values will be returned:
SPI_OK_UTILITY if some utility (e.g. CREATE TABLE ...) was executed.
SPI_OK_SELECT if SELECT (but not SELECT ... INTO!) was executed.
SPI_OK_SELINTO if SELECT ... INTO was executed.
SPI_OK_INSERT if INSERT (or INSERT ... SELECT) was executed.
SPI_OK_DELETE if DELETE was executed.
SPI_OK_UPDATE if UPDATE was executed.
NOTE! You may pass many queries in one string or query string may be
re-written by RULEs. SPI_exec() returns result for last query executed.
Actual number of tuples for which (last) query was executed is returned
in global variable SPI_processed (if not SPI_OK_UTILITY).
If SPI_OK_SELECT returned and SPI_processed > 0 then you may use global
pointer SPITupleTable *SPI_tuptable to access selected tuples:
Structure SPITupleTable is defined in spi.h:
typedef struct
{
uint32 alloced; /* # of alloced vals */
uint32 free; /* # of free vals */
TupleDesc tupdesc; /* tuple descriptor */
HeapTuple *vals; /* tuples */
} SPITupleTable;
HeapTuple *vals is array of pointers to tuples. TupleDesc tupdesc is
tuple descriptor which you are to pass to SPI functions dealing with
tuples.
NOTE! Functions SPI_exec(), SPI_execp() and SPI_prepare() change both
SPI_processed and SPI_tuptable (just pointer, not context of structure)!
So, save theme in local procedure variables if you need.
Also NOTE, that SPI_finish() frees and makes all SPITupleTable-s
unusable! (See Memory management).
SPI_exec() may return one of the next (negative) values:
SPI_ERROR_ARGUMENT if query is NULL or tcount < 0.
SPI_ERROR_UNCONNECTED if procedure is un-connected.
SPI_ERROR_COPY if COPY TO/FROM stdin.
SPI_ERROR_CURSOR if DECLARE/CLOSE CURSOR, FETCH.
SPI_ERROR_TRANSACTION if BEGIN/ABORT/END.
SPI_ERROR_OPUNKNOWN if type of query is unknown (this shouldn't occure).
void *SPI_prepare(char *query, int nargs, Oid * argtypes)
Creates and returns execution plan (parser+planner+optimizer) but doesn't
execute query. Should be called from connected procedure.
nargs is number of parameters ($1 ... $<nargs> - like in SQL-functions),
*argtypes is array of parameter type OIDs.
nargs may be 0 only if there is no any $1 in query.
Execution of prepared execution plans is much faster sometimes... So this
feature may be useful if the same query will be executed may times.
NOTE! Plan returned by SPI_prepare() may be used only in current
invocation of procedure: SPI_finish() frees memory allocated for a plan.
See SPI_saveplan().
If successful, NOT NULL pointer will be returned. Otherwise, you'll get
NULL plan. In both cases SPI_result will be setted like value returned by
SPI_exec, but
SPI_ERROR_ARGUMENT if query is NULL or nargs < 0 or nargs > 0 && argtypes
is NULL.
void *SPI_saveplan(void *plan)
Currently, there is no ability to store prepared plans in system catalog
and fetch them from there for execution. This will be implemented in
future versions.
As work arround, there is ability to re-use prepared plans in the
consequent invocations of your procedure in current session.
SPI_saveplan() saves passed plan (prepared by SPI_prepare()) in memory
protected from free-ing by SPI_finish() and by transaction manager and
returns pointer to saved plan. You may preserve pointer returned in local
variable and always check is this pointer NULL or not to either prepare
plan or use already prepared plan in SPI_execp (see below).
NOTE! If one of objects (relation, function, ...) referenced by prepared
plan will be dropped during your session (by your or another backend)
then results of SPI_execp (for this plan) will be unpredictable.
If successful, NOT NULL returned. Otherwise, SPI_result setted to
SPI_ERROR_ARGUMENT if plan is NULL.
SPI_ERROR_UNCONNECTED if procedure is un-connected.
int SPI_execp(void *plan, Datum * values, char *Nulls, int tcount)
Executes plan prepared by SPI_prepare() (or returned by SPI_saveplan()).
Should be called from connected procedure.
plan is pointer to execution plan, values points to actual parameter
values, Nulls - to array describing what parameters get NULLs ('n' -
NULL, ' ' - NOT NULL), tcount - number of tuples for which plan is to be
executed.
If Nulls is NULL then SPI assumes that all values (if any) are NOT NULL.
Returns value like SPI_exec, but
SPI_ERROR_ARGUMENT if plan is NULL or tcount < 0.
SPI_ERROR_PARAM if Values is NULL and plan prepared with some parameters.
If successful, SPI_tuptable and SPI_processed are initialized like by
SPI_exec().
All functions described below may be used by connected and un-connected
procedures.
HeapTuple SPI_copytuple(HeapTuple tuple)
Makes copy of tuple in upper Executor context (see Memory management).
If successful, NOT NULL returned. NULL (i.e. - error) will be returned
only if NULL passed in.
HeapTuple SPI_modifytuple(Relation rel, HeapTuple tuple, int natts,
int *attnum, Datum * Values, char *Nulls)
Modifies tuple of relation rel as described by the rest of arguments.
natts is number of attribute numbers in attnum.
attnum is array of numbers of attributes which are to be changed.
Values are new values for attributes specified.
Nulls describes what of attributes specified are NULL (if Nulls is
NULL then no NULLs).
If successful, NOT NULL pointer to new tuple returned. New tuple is
allocated in upper Executor context (see Memory management). Passed tuple
is not changed.
Returns NULL if failed and cause in SPI_result:
SPI_ERROR_ARGUMENT if rel is NULL or tuple is NULL or natts le 0 or
attnum is NULL or Values is NULL.
SPI_ERROR_NOATTRIBUTE if there is invalid (le 0 or gt number of
attributes in tuple) attribute number in attnum.
int SPI_fnumber(TupleDesc tupdesc, char *fname)
Returns attribute number for attribute with name as in fname.
tupdesc is tuple description.
Attribute numbers are 1-based.
Returns SPI_ERROR_NOATTRIBUTE if attribute not found.
char *SPI_fname(TupleDesc tupdesc, int fnumber)
Returns (copy of) name of attribute with number fnumber.
Returns NULL and (SPI_ERROR_NOATTRIBUTE in SPI_result) if fnumber is
greater number of attributes in tupdesc or fnumber le 0.
char *SPI_getvalue(HeapTuple tuple, TupleDesc tupdesc, int fnumber)
Returns external (string) representation of value of attribute fnumber in
tuple with descriptor tupdesc. Allocates memory as required by value.
Returns NULL if
attribute is NULL (SPI_result is 0 - no error);
fnumber is invalid (SPI_result is SPI_ERROR_NOATTRIBUTE);
there is no output function (SPI_result is SPI_ERROR_NOOUTFUNC).
Datum SPI_getbinval(HeapTuple tuple, TupleDesc tupdesc, int fnumber,
bool *isnull)
Returns value of attribute fnumber in tuple with descriptor tupdesc. This
is binary value in internal form. This is not copy!
Returns NULL indicator in *isnull.
SPI_result is SPI_ERROR_NOATTRIBUTE if fnumber is invalid.
char *SPI_gettype(TupleDesc tupdesc, int fnumber)
Returns (copy of) type name for attribute fnumber.
Returns NULL (and SPI_ERROR_NOATTRIBUTE in SPI_result) if fnumber
is invalid.
Oid SPI_gettypeid(TupleDesc tupdesc, int fnumber)
Returns type OID for attribute fnumber.
SPI_result is SPI_ERROR_NOATTRIBUTE if fnumber is invalid.
char *SPI_getrelname(Relation rel)
Returns (copy of) relation name of relation rel.
void *SPI_palloc (Size size)
Allocates memory in upper Executor context (see Memory management).
void *SPI_repalloc(void *pointer, Size size)
Re-allocates memory allocated in upper Executor context (see Memory
management).
void SPI_pfree(void *pointer)
Frees memory allocated in upper Executor context (see Memory management).
Memory management
Server allocates memory in memory contexts in such way that allocations
made in one context may be freed by context destruction without affecting
allocations made in other contexts. There is way to choose some context as
current one. All allocations (via palloc(), etc) are made in current
context. You'll get unpredictable results if you'll try to free (or
reallocate) memory allocated not in current context.
SPI procedures deal with two memory contexts: upper Executor memory
context and procedure memory context (if connected).
Before a procedure is connected to SPI manager current memory context is
upper Executor context. And so, all allocation made by procedure itself via
palloc()/repalloc() or by SPI utility functions before connection to SPI are
made in this context.
After SPI_connect() is called current context is procedure one. All
allocations made via palloc()/repalloc() or by SPI utility functions (except
for SPI_copytuple(), SPI_modifytuple, SPI_palloc() and SPI_repalloc()) are
made in this context.
When a procedure dis-connects from SPI manager (via SPI_finish()) current
context is restored to upper Executor context and all allocations made in
procedure memory context are freed and can't be used any more!
If you want to return something to upper Executor then you have to
allocate memory for this in upper context!
SPI has no ability to automatically free allocations in upper Executor
context!
SPI automatically frees memory allocated during execution of a query when
this query is done!
Examples
There are complex examples in contrib/spi and in
src/test/regress/regress.c.
This is very simple example of SPI using. Function execq accepts
SQL-query in first arguments and tcount in second, executes query
using SPI_exec and returns number of tuples for which query executed:
----------------------------------------------------------------------------
#include "executor/spi.h" /* this is what you need to work with SPI */
int execq(text *sql, int cnt);
int
execq(text *sql, int cnt)
{
int ret;
int proc = 0;
SPI_connect();
ret = SPI_exec(textout(sql), cnt);
proc = SPI_processed;
/*
* If this is SELECT and some tuple(s) fetched -
* returns tuples to the caller via elog (NOTICE).
*/
if ( ret == SPI_OK_SELECT && SPI_processed > 0 )
{
TupleDesc tupdesc = SPI_tuptable->tupdesc;
SPITupleTable *tuptable = SPI_tuptable;
char buf[8192];
int i;
for (ret = 0; ret < proc; ret++)
{
HeapTuple tuple = tuptable->vals[ret];
for (i = 1, buf[0] = 0; i <= tupdesc->natts; i++)
sprintf(buf + strlen (buf), " %s%s",
SPI_getvalue(tuple, tupdesc, i),
(i == tupdesc->natts) ? " " : " |");
elog (NOTICE, "EXECQ: %s", buf);
}
}
SPI_finish();
return (proc);
}
----------------------------------------------------------------------------
Now, compile and create function:
create function execq (text, int4) returns int4 as '...path_to_so' language 'c';
vac=> select execq('create table a (x int4)', 0);
execq
-----
0
(1 row)
vac=> insert into a values (execq('insert into a values (0)',0));
INSERT 167631 1
vac=> select execq('select * from a',0);
NOTICE:EXECQ: 0 <<< inserted by execq
NOTICE:EXECQ: 1 <<< value returned by execq and inserted by upper INSERT
execq
-----
2
(1 row)
vac=> select execq('insert into a select x + 2 from a',1);
execq
-----
1
(1 row)
vac=> select execq('select * from a', 10);
NOTICE:EXECQ: 0
NOTICE:EXECQ: 1
NOTICE:EXECQ: 2 <<< 0 + 2, only one tuple inserted - as specified
execq
-----
3 <<< 10 is max value only, 3 is real # of tuples
(1 row)