Proposal for function-manager redesign 19-Nov-2000 -------------------------------------- We know that the existing mechanism for calling Postgres functions needs to be redesigned. It has portability problems because it makes assumptions about parameter passing that violate ANSI C; it fails to handle NULL arguments and results cleanly; and "function handlers" that support a class of functions (such as fmgr_pl) can only be done via a really ugly, non-reentrant kluge. (Global variable set during every function call, forsooth.) Here is a proposal for fixing these problems. In the past, the major objections to redoing the function-manager interface have been (a) it'll be quite tedious to implement, since every built-in function and everyplace that calls such functions will need to be touched; (b) such wide-ranging changes will be difficult to make in parallel with other development work; (c) it will break existing user-written loadable modules that define "C language" functions. While I have no solution to the "tedium" aspect, I believe I see an answer to the other problems: by use of function handlers, we can support both old and new interfaces in parallel for both callers and callees, at some small efficiency cost for the old styles. That way, most of the changes can be done on an incremental file-by-file basis --- we won't need a "big bang" where everything changes at once. Support for callees written in the old style can be left in place indefinitely, to provide backward compatibility for user-written C functions. Changes in pg_proc (system data about a function) ------------------------------------------------- A new column "proisstrict" will be added to the system pg_proc table. This is a boolean value which will be TRUE if the function is "strict", that is it always returns NULL when any of its inputs are NULL. The function manager will check this field and skip calling the function when it's TRUE and there are NULL inputs. This allows us to remove explicit NULL-value tests from many functions that currently need them (not to mention fixing many more that need them but don't have them). A function that is not marked "strict" is responsible for checking whether its inputs are NULL or not. Most builtin functions will be marked "strict". An optional WITH parameter will be added to CREATE FUNCTION to allow specification of whether user-defined functions are strict or not. I am inclined to make the default be "not strict", since that seems to be the more useful case for functions expressed in SQL or a PL language, but am open to arguments for the other choice. The new function-manager interface ---------------------------------- The core of the new design is revised data structures for representing the result of a function lookup and for representing the parameters passed to a specific function invocation. (We want to keep function lookup separate from function call, since many parts of the system apply the same function over and over; the lookup overhead should be paid once per query, not once per tuple.) When a function is looked up in pg_proc, the result is represented as typedef struct { PGFunction fn_addr; /* pointer to function or handler to be called */ Oid fn_oid; /* OID of function (NOT of handler, if any) */ short fn_nargs; /* 0..FUNC_MAX_ARGS, or -1 if variable arg count */ bool fn_strict; /* function is "strict" (NULL in => NULL out) */ bool fn_retset; /* function returns a set (over multiple calls) */ void *fn_extra; /* extra space for use by handler */ MemoryContext fn_mcxt; /* memory context to store fn_extra in */ Node *fn_expr; /* expression parse tree for call, or NULL */ } FmgrInfo; For an ordinary built-in function, fn_addr is just the address of the C routine that implements the function. Otherwise it is the address of a handler for the class of functions that includes the target function. The handler can use the function OID and perhaps also the fn_extra slot to find the specific code to execute. (fn_oid = InvalidOid can be used to denote a not-yet-initialized FmgrInfo struct. fn_extra will always be NULL when an FmgrInfo is first filled by the function lookup code, but a function handler could set it to avoid making repeated lookups of its own when the same FmgrInfo is used repeatedly during a query.) fn_nargs is the number of arguments expected by the function, fn_strict is its strictness flag, and fn_retset shows whether it returns a set; all of these values come from the function's pg_proc entry. If the function is being called as part of a SQL expression, fn_expr will point to the expression parse tree for the function call; this can be used to extract parse-time knowledge about the actual arguments. FmgrInfo already exists in the current code, but has fewer fields. This change should be transparent at the source-code level. During a call of a function, the following data structure is created and passed to the function: typedef struct { FmgrInfo *flinfo; /* ptr to lookup info used for this call */ Node *context; /* pass info about context of call */ Node *resultinfo; /* pass or return extra info about result */ bool isnull; /* function must set true if result is NULL */ short nargs; /* # arguments actually passed */ Datum arg[FUNC_MAX_ARGS]; /* Arguments passed to function */ bool argnull[FUNC_MAX_ARGS]; /* T if arg[i] is actually NULL */ } FunctionCallInfoData; typedef FunctionCallInfoData* FunctionCallInfo; flinfo points to the lookup info used to make the call. Ordinary functions will probably ignore this field, but function class handlers will need it to find out the OID of the specific function being called. context is NULL for an "ordinary" function call, but may point to additional info when the function is called in certain contexts. (For example, the trigger manager will pass information about the current trigger event here.) If context is used, it should point to some subtype of Node; the particular kind of context is indicated by the node type field. (A callee should always check the node type before assuming it knows what kind of context is being passed.) fmgr itself puts no other restrictions on the use of this field. resultinfo is NULL when calling any function from which a simple Datum result is expected. It may point to some subtype of Node if the function returns more than a Datum. (For example, resultinfo is used when calling a function that returns a set, as discussed below.) Like the context field, resultinfo is a hook for expansion; fmgr itself doesn't constrain the use of the field. nargs, arg[], and argnull[] hold the arguments being passed to the function. Notice that all the arguments passed to a function (as well as its result value) will now uniformly be of type Datum. As discussed below, callers and callees should apply the standard Datum-to-and-from-whatever macros to convert to the actual argument types of a particular function. The value in arg[i] is unspecified when argnull[i] is true. It is generally the responsibility of the caller to ensure that the number of arguments passed matches what the callee is expecting; except for callees that take a variable number of arguments, the callee will typically ignore the nargs field and just grab values from arg[]. The isnull field will be initialized to "false" before the call. On return from the function, isnull is the null flag for the function result: if it is true the function's result is NULL, regardless of the actual function return value. Note that simple "strict" functions can ignore both isnull and argnull[], since they won't even get called when there are any TRUE values in argnull[]. FunctionCallInfo replaces FmgrValues plus a bunch of ad-hoc parameter conventions, global variables (fmgr_pl_finfo and CurrentTriggerData at least), and other uglinesses. Callees, whether they be individual functions or function handlers, shall always have this signature: Datum function (FunctionCallInfo fcinfo); which is represented by the typedef typedef Datum (*PGFunction) (FunctionCallInfo fcinfo); The function is responsible for setting fcinfo->isnull appropriately as well as returning a result represented as a Datum. Note that since all callees will now have exactly the same signature, and will be called through a function pointer declared with exactly that signature, we should have no portability or optimization problems. Function coding conventions --------------------------- As an example, int4 addition goes from old-style int32 int4pl(int32 arg1, int32 arg2) { return arg1 + arg2; } to new-style Datum int4pl(FunctionCallInfo fcinfo) { /* we assume the function is marked "strict", so we can ignore * NULL-value handling */ return Int32GetDatum(DatumGetInt32(fcinfo->arg[0]) + DatumGetInt32(fcinfo->arg[1])); } This is, of course, much uglier than the old-style code, but we can improve matters with some well-chosen macros for the boilerplate parts. I propose below macros that would make the code look like Datum int4pl(PG_FUNCTION_ARGS) { int32 arg1 = PG_GETARG_INT32(0); int32 arg2 = PG_GETARG_INT32(1); PG_RETURN_INT32( arg1 + arg2 ); } This is still more code than before, but it's fairly readable, and it's also amenable to machine processing --- for example, we could probably write a script that scans code like this and extracts argument and result type info for comparison to the pg_proc table. For the standard data types float4, float8, and int8, these macros should hide the indirection and space allocation involved, so that the function's code is not explicitly aware that these types are pass-by-reference. This will offer a considerable gain in readability, and it also opens up the opportunity to make these types be pass-by-value on machines where it's feasible to do so. (For example, on an Alpha it's pretty silly to make int8 be pass-by-ref, since Datum is going to be 64 bits anyway. float4 could become pass-by-value on all machines...) Here are the proposed macros and coding conventions: The definition of an fmgr-callable function will always look like Datum function_name(PG_FUNCTION_ARGS) { ... } "PG_FUNCTION_ARGS" just expands to "FunctionCallInfo fcinfo". The main reason for using this macro is to make it easy for scripts to spot function definitions. However, if we ever decide to change the calling convention again, it might come in handy to have this macro in place. A nonstrict function is responsible for checking whether each individual argument is null or not, which it can do with PG_ARGISNULL(n) (which is just "fcinfo->argnull[n]"). It should avoid trying to fetch the value of any argument that is null. Both strict and nonstrict functions can return NULL, if needed, with PG_RETURN_NULL(); which expands to { fcinfo->isnull = true; return (Datum) 0; } Argument values are ordinarily fetched using code like int32 name = PG_GETARG_INT32(number); For float4, float8, and int8, the PG_GETARG macros will hide the pass-by- reference nature of the data types; for example PG_GETARG_FLOAT4 expands to (* (float4 *) DatumGetPointer(fcinfo->arg[number])) and would typically be called like this: float4 arg = PG_GETARG_FLOAT4(0); Note that "float4" and "float8" are the recommended typedefs to use, not "float32data" and "float64data", and the macros are named accordingly. But 64-bit ints should be declared as "int64". Non-null values are returned with a PG_RETURN_XXX macro of the appropriate type. For example, PG_RETURN_INT32 expands to return Int32GetDatum(x) PG_RETURN_FLOAT4, PG_RETURN_FLOAT8, and PG_RETURN_INT64 hide the pass-by- reference nature of their datatypes. fmgr.h will provide PG_GETARG and PG_RETURN macros for all the basic data types. Modules or header files that define specialized SQL datatypes (eg, timestamp) should define appropriate macros for those types, so that functions manipulating the types can be coded in the standard style. For non-primitive data types (particularly variable-length types) it won't be very practical to hide the pass-by-reference nature of the data type, so the PG_GETARG and PG_RETURN macros for those types won't do much more than DatumGetPointer/PointerGetDatum plus the appropriate typecast (but see TOAST discussion, below). Functions returning such types will need to palloc() their result space explicitly. I recommend naming the GETARG and RETURN macros for such types to end in "_P", as a reminder that they produce or take a pointer. For example, PG_GETARG_TEXT_P yields "text *". When a function needs to access fcinfo->flinfo or one of the other auxiliary fields of FunctionCallInfo, it should just do it. I doubt that providing syntactic-sugar macros for these cases is useful. Call-site coding conventions ---------------------------- There are many places in the system that call either a specific function (for example, the parser invokes "textin" by name in places) or a particular group of functions that have a common argument list (for example, the optimizer invokes selectivity estimation functions with a fixed argument list). These places will need to change, but we should try to avoid making them significantly uglier than before. Places that invoke an arbitrary function with an arbitrary argument list can simply be changed to fill a FunctionCallInfoData structure directly; that'll be no worse and possibly cleaner than what they do now. When invoking a specific built-in function by name, we have generally just written something like result = textin ( ... args ... ) which will not work after textin() is converted to the new call style. I suggest that code like this be converted to use "helper" functions that will create and fill in a FunctionCallInfoData struct. For example, if textin is being called with one argument, it'd look something like result = DirectFunctionCall1(textin, PointerGetDatum(argument)); These helper routines will have declarations like Datum DirectFunctionCall2(PGFunction func, Datum arg1, Datum arg2); Note it will be the caller's responsibility to convert to and from Datum; appropriate conversion macros should be used. The DirectFunctionCallN routines will not bother to fill in fcinfo->flinfo (indeed cannot, since they have no idea about an OID for the target function); they will just set it NULL. This is unlikely to bother any built-in function that could be called this way. Note also that this style of coding cannot pass a NULL input value nor cope with a NULL result (it couldn't before, either!). We can make the helper routines elog an error if they see that the function returns a NULL. When invoking a function that has a known argument signature, we have usually written either result = fmgr(targetfuncOid, ... args ... ); or result = fmgr_ptr(FmgrInfo *finfo, ... args ... ); depending on whether an FmgrInfo lookup has been done yet or not. This kind of code can be recast using helper routines, in the same style as above: result = OidFunctionCall1(funcOid, PointerGetDatum(argument)); result = FunctionCall2(funcCallInfo, PointerGetDatum(argument), Int32GetDatum(argument)); Again, this style of coding does not allow for expressing NULL inputs or receiving a NULL result. As with the callee-side situation, I propose adding argument conversion macros that hide the pass-by-reference nature of int8, float4, and float8, with an eye to making those types relatively painless to convert to pass-by-value. The existing helper functions fmgr(), fmgr_c(), etc will be left in place until all uses of them are gone. Of course their internals will have to change in the first step of implementation, but they can continue to support the same external appearance. Support for TOAST-able data types --------------------------------- For TOAST-able data types, the PG_GETARG macro will deliver a de-TOASTed data value. There might be a few cases where the still-toasted value is wanted, but the vast majority of cases want the de-toasted result, so that will be the default. To get the argument value without causing de-toasting, use PG_GETARG_RAW_VARLENA_P(n). Some functions require a modifiable copy of their input values. In these cases, it's silly to do an extra copy step if we copied the data anyway to de-TOAST it. Therefore, each toastable datatype has an additional fetch macro, for example PG_GETARG_TEXT_P_COPY(n), which delivers a guaranteed-fresh copy, combining this with the detoasting step if possible. There is also a PG_FREE_IF_COPY(ptr,n) macro, which pfree's the given pointer if and only if it is different from the original value of the n'th argument. This can be used to free the de-toasted value of the n'th argument, if it was actually de-toasted. Currently, doing this is not necessary for the majority of functions because the core backend code releases temporary space periodically, so that memory leaked in function execution isn't a big problem. However, as of 7.1 memory leaks in functions that are called by index searches will not be cleaned up until end of transaction. Therefore, functions that are listed in pg_amop or pg_amproc should be careful not to leak detoasted copies, and so these functions do need to use PG_FREE_IF_COPY() for toastable inputs. A function should never try to re-TOAST its result value; it should just deliver an untoasted result that's been palloc'd in the current memory context. When and if the value is actually stored into a tuple, the tuple toaster will decide whether toasting is needed. Functions accepting or returning sets ------------------------------------- [ this section revised 29-Aug-2002 for 7.3 ] If a function is marked in pg_proc as returning a set, then it is called with fcinfo->resultinfo pointing to a node of type ReturnSetInfo. A function that desires to return a set should raise an error "called in context that does not accept a set result" if resultinfo is NULL or does not point to a ReturnSetInfo node. There are currently two modes in which a function can return a set result: value-per-call, or materialize. In value-per-call mode, the function returns one value each time it is called, and finally reports "done" when it has no more values to return. In materialize mode, the function's output set is instantiated in a Tuplestore object; all the values are returned in one call. Additional modes might be added in future. ReturnSetInfo contains a field "allowedModes" which is set (by the caller) to a bitmask that's the OR of the modes the caller can support. The actual mode used by the function is returned in another field "returnMode". For backwards-compatibility reasons, returnMode is initialized to value-per-call and need only be changed if the function wants to use a different mode. The function should elog() if it cannot use any of the modes the caller is willing to support. Value-per-call mode works like this: ReturnSetInfo contains a field "isDone", which should be set to one of these values: ExprSingleResult /* expression does not return a set */ ExprMultipleResult /* this result is an element of a set */ ExprEndResult /* there are no more elements in the set */ (the caller will initialize it to ExprSingleResult). If the function simply returns a Datum without touching ReturnSetInfo, then the call is over and a single-item set has been returned. To return a set, the function must set isDone to ExprMultipleResult for each set element. After all elements have been returned, the next call should set isDone to ExprEndResult and return a null result. (Note it is possible to return an empty set by doing this on the first call.) The ReturnSetInfo node also contains a link to the ExprContext within which the function is being evaluated. This is useful for value-per-call functions that need to close down internal state when they are not run to completion: they can register a shutdown callback function in the ExprContext. Materialize mode works like this: the function creates a Tuplestore holding the (possibly empty) result set, and returns it. There are no multiple calls. The function must also return a TupleDesc that indicates the tuple structure. The Tuplestore and TupleDesc should be created in the context econtext->ecxt_per_query_memory (note this will *not* be the context the function is called in). The function stores pointers to the Tuplestore and TupleDesc into ReturnSetInfo, sets returnMode to indicate materialize mode, and returns null. isDone is not used and should be left at ExprSingleResult. If the function is being called as a table function (ie, it appears in a FROM item), then the expected tuple descriptor is passed in ReturnSetInfo; in other contexts the expectedDesc field will be NULL. The function need not pay attention to expectedDesc, but it may be useful in special cases. There is no support for functions accepting sets; instead, the function will be called multiple times, once for each element of the input set. Notes about function handlers ----------------------------- Handlers for classes of functions should find life much easier and cleaner in this design. The OID of the called function is directly reachable from the passed parameters; we don't need the global variable fmgr_pl_finfo anymore. Also, by modifying fcinfo->flinfo->fn_extra, the handler can cache lookup info to avoid repeat lookups when the same function is invoked many times. (fn_extra can only be used as a hint, since callers are not required to re-use an FmgrInfo struct. But in performance-critical paths they normally will do so.) If the handler wants to allocate memory to hold fn_extra data, it should NOT do so in CurrentMemoryContext, since the current context may well be much shorter-lived than the context where the FmgrInfo is. Instead, allocate the memory in context flinfo->fn_mcxt, or in a long-lived cache context. fn_mcxt normally points at the context that was CurrentMemoryContext at the time the FmgrInfo structure was created; in any case it is required to be a context at least as long-lived as the FmgrInfo itself. Telling the difference between old- and new-style functions ----------------------------------------------------------- During the conversion process, we carried two different pg_language entries, "internal" and "newinternal", for internal functions. The function manager used the language code to distinguish which calling convention to use. (Old-style internal functions were supported via a function handler.) As of Nov. 2000, no old-style internal functions remain, so we can drop support for them. We will remove the old "internal" pg_language entry and rename "newinternal" to "internal". The interim solution for dynamically-loaded compiled functions has been similar: two pg_language entries "C" and "newC". This naming convention is not desirable for the long run, and yet we cannot stop supporting old-style user functions. Instead, it seems better to use just one pg_language entry "C", and require the dynamically-loaded library to provide additional information that identifies new-style functions. This avoids compatibility problems --- for example, existing dump scripts will identify PL language handlers as being in language "C", which would be wrong under the "newC" convention. Also, this approach should generalize more conveniently for future extensions to the function interface specification. Given a dynamically loaded function named "foo" (note that the name being considered here is the link-symbol name, not the SQL-level function name), the function manager will look for another function in the same dynamically loaded library named "pg_finfo_foo". If this second function does not exist, then foo is assumed to be called old-style, thus ensuring backwards compatibility with existing libraries. If the info function does exist, it is expected to have the signature Pg_finfo_record * pg_finfo_foo (void); The info function will be called by the fmgr, and must return a pointer to a Pg_finfo_record struct. (The returned struct will typically be a statically allocated constant in the dynamic-link library.) The current definition of the struct is just typedef struct { int api_version; } Pg_finfo_record; where api_version is 0 to indicate old-style or 1 to indicate new-style calling convention. In future releases, additional fields may be defined after api_version, but these additional fields will only be used if api_version is greater than 1. These details will be hidden from the author of a dynamically loaded function by using a macro. To define a new-style dynamically loaded function named foo, write PG_FUNCTION_INFO_V1(foo); Datum foo(PG_FUNCTION_ARGS) { ... } The function itself is written using the same conventions as for new-style internal functions; you just need to add the PG_FUNCTION_INFO_V1() macro. Note that old-style and new-style functions can be intermixed in the same library, depending on whether or not you write a PG_FUNCTION_INFO_V1() for each one. The SQL declaration for a dynamically-loaded function is CREATE FUNCTION foo ... LANGUAGE 'C' regardless of whether it is old- or new-style. New-style dynamic functions will be invoked directly by fmgr, and will therefore have the same performance as internal functions after the initial pg_proc lookup overhead. Old-style dynamic functions will be invoked via a handler, and will therefore have a small performance penalty. To allow old-style dynamic functions to work safely on toastable datatypes, the handler for old-style functions will automatically detoast toastable arguments before passing them to the old-style function. A new-style function is expected to take care of toasted arguments by using the standard argument access macros defined above.