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 may 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 0, then the number of rows for which the command will be executed is restricted (much like a LIMIT clause). For example,
SPI_execute("INSERT INTO foo SELECT * FROM bar", false, 5);
will allow at most 5 rows to be inserted into the table.
You may pass multiple commands in one string. SPI_execute returns the result for the command executed last. The count limit applies to each command separately, but it is not applied to 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 (unless the return value of the function is SPI_OK_UTILITY). If the return value of the function is SPI_OK_SELECT then you may use the global pointer SPITupleTable *SPI_tuptable to access the result rows.
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 may 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 process or return
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 a DELETE was executed
if an INSERT was executed
if an UPDATE was executed
if a utility command (e.g., CREATE TABLE) was executed
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 DECLARE, CLOSE, or FETCH was attempted
if BEGIN, COMMIT, or ROLLBACK was attempted
if the command type is unknown (shouldn't happen)
if called from an unconnected procedure
The functions SPI_execute, SPI_exec, SPI_execute_plan, and SPI_execp change 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 a related function across later calls.
