SPI_execute executes the
specified SQL command for count rows.
If read_only is true, the command must be read-only, and execution
overhead is somewhat reduced.
This function can only be called from a connected procedure.
If count is zero then the command is executed for all rows that it applies to. If count is greater than zero, then no more than count rows will be retrieved; execution stops when the count is reached, much like adding a LIMIT clause to the query. For example,
SPI_execute("SELECT * FROM foo", true, 5);
will retrieve at most 5 rows from the table. Note that such a limit is only effective when the command actually returns rows. For example,
SPI_execute("INSERT INTO foo SELECT * FROM bar", false, 5);
inserts all rows from bar, ignoring the count parameter. However, with
SPI_execute("INSERT INTO foo SELECT * FROM bar RETURNING *", false, 5);
at most 5 rows would be inserted, since execution would stop after the fifth RETURNING result row is retrieved.
You can pass multiple commands in one string, but later
commands cannot depend on the creation of objects earlier in the
string, because the whole string will be parsed and planned
before execution begins. SPI_execute returns the result for the command
executed last. The count limit applies
to each command separately (even though only the last result will
actually be returned). The limit is not applied to any hidden
commands generated by rules.
When read_only is false, SPI_execute
increments the command counter and computes a new snapshot before executing each command in the
string. The snapshot does not actually change if the current
transaction isolation level is SERIALIZABLE, but in READ
COMMITTED mode the snapshot update allows each command to
see the results of newly committed transactions from other
sessions. This is essential for consistent behavior when the
commands are modifying the database.
When read_only is true, SPI_execute
does not update either the snapshot or the command counter, and
it allows only plain SELECT commands to
appear in the command string. The commands are executed using the
snapshot previously established for the surrounding query. This
execution mode is somewhat faster than the read/write mode due to
eliminating per-command overhead. It also allows genuinely
stable functions to be built: since
successive executions will all use the same snapshot, there will
be no change in the results.
It is generally unwise to mix read-only and read-write commands within a single function using SPI; that could result in very confusing behavior, since the read-only queries would not see the results of any database updates done by the read-write queries.
The actual number of rows for which the (last) command was executed is returned in the global variable SPI_processed. If the return value of the function is SPI_OK_SELECT, SPI_OK_INSERT_RETURNING, SPI_OK_DELETE_RETURNING, or SPI_OK_UPDATE_RETURNING, then you can use the global pointer SPITupleTable *SPI_tuptable to access the result rows. Some utility commands (such as EXPLAIN) also return row sets, and SPI_tuptable will contain the result in these cases too.
The structure SPITupleTable is defined thus:
typedef struct
{
MemoryContext tuptabcxt; /* memory context of result table */
uint32 alloced; /* number of alloced vals */
uint32 free; /* number of free vals */
TupleDesc tupdesc; /* row descriptor */
HeapTuple *vals; /* rows */
} SPITupleTable;
vals is an array of pointers to rows. (The number of valid entries is given by SPI_processed.) tupdesc is a row descriptor which you can pass to SPI functions dealing with rows. tuptabcxt, alloced, and free are internal fields not intended for use by SPI callers.
SPI_finish frees all SPITupleTables allocated during the current
procedure. You can free a particular result table earlier, if you
are done with it, by calling SPI_freetuptable.
string containing command to execute
true for read-only execution
maximum number of rows to return, or 0 for no limit
If the execution of the command was successful then one of the following (nonnegative) values will be returned:
if a SELECT (but not SELECT INTO) was executed
if a SELECT INTO was executed
if an INSERT was executed
if a DELETE was executed
if an UPDATE was executed
if an INSERT RETURNING was executed
if a DELETE RETURNING was executed
if an UPDATE RETURNING was executed
if a utility command (e.g., CREATE TABLE) was executed
if the command was rewritten into another kind of command (e.g., UPDATE became an INSERT) by a rule.
On error, one of the following negative values is returned:
if command is NULL or count is less than 0
if COPY TO stdout or COPY FROM stdin was attempted
if a transaction manipulation command was attempted (BEGIN, COMMIT, ROLLBACK, SAVEPOINT, PREPARE TRANSACTION, COMMIT PREPARED, ROLLBACK PREPARED, or any variant thereof)
if the command type is unknown (shouldn't happen)
if called from an unconnected procedure
All SPI query-execution functions set both SPI_processed and SPI_tuptable (just the pointer, not the contents
of the structure). Save these two global variables into local
procedure variables if you need to access the result table of
SPI_execute or another
query-execution function across later calls.