Use standard casting mechanism to convert types in plpgsql, when possible.

plpgsql's historical method for converting datatypes during assignments was
to apply the source type's output function and then the destination type's
input function.  Aside from being miserably inefficient in most cases, this
method failed outright in many cases where a user might expect it to work;
an example is that "declare x int; ... x := 3.9;" would fail, not round the
value to 4.

Instead, let's convert by applying the appropriate assignment cast whenever
there is one.  To avoid breaking compatibility unnecessarily, fall back to
the I/O conversion method if there is no assignment cast.

So far as I can tell, there is just one case where this method produces a
different result than the old code in a case where the old code would not
have thrown an error.  That is assignment of a boolean value to a string
variable (type text, varchar, or bpchar); the old way gave boolean's output
representation, ie 't'/'f', while the new way follows the behavior of the
bool-to-text cast and so gives 'true' or 'false'.  This will need to be
called out as an incompatibility in the 9.5 release notes.

Aside from handling many conversion cases more sanely, this method is
often significantly faster than the old way.  In part that's because
of more effective caching of the conversion info.

doc/src/sgml/plpgsql.sgml

@@ -881,13 +881,14 @@ PREPARE <replaceable>statement_name</>(integer, integer) AS SELECT $1 &lt; $2;
 
     <para>
      If the expression's result data type doesn't match the variable's
-     data type, or the variable has a specific size/precision
-     (like <type>char(20)</type>), the result value will be implicitly
-     converted by the <application>PL/pgSQL</application> interpreter using
-     the result type's output-function and
-     the variable type's input-function. Note that this could potentially
-     result in run-time errors generated by the input function, if the
-     string form of the result value is not acceptable to the input function.
+     data type, the value will be coerced as though by an assignment cast
+     (see <xref linkend="typeconv-query">).  If no assignment cast is known
+     for the pair of data types involved, the <application>PL/pgSQL</>
+     interpreter will attempt to convert the result value textually, that is
+     by applying the result type's output function followed by the variable
+     type's input function.  Note that this could result in run-time errors
+     generated by the input function, if the string form of the result value
+     is not acceptable to the input function.
     </para>
 
     <para>

doc/src/sgml/typeconv.sgml

@@ -844,9 +844,10 @@ Check for an exact match with the target.
 
 <step performance="required">
 <para>
-Otherwise, try to convert the expression to the target type.  This will succeed
-if there is a registered cast between the two types.
-If the expression is an unknown-type literal, the contents of
+Otherwise, try to convert the expression to the target type.  This is possible
+if an <firstterm>assignment cast</> between the two types is registered in the
+<structname>pg_cast</> catalog (see <xref linkend="sql-createcast">).
+Alternatively, if the expression is an unknown-type literal, the contents of
 the literal string will be fed to the input conversion routine for the target
 type.
 </para>

src/pl/plpgsql/src/pl_comp.c

@@ -559,8 +559,6 @@ do_compile(FunctionCallInfo fcinfo,
 			{
 				function->fn_retbyval = typeStruct->typbyval;
 				function->fn_rettyplen = typeStruct->typlen;
-				function->fn_rettypioparam = getTypeIOParam(typeTup);
-				fmgr_info(typeStruct->typinput, &(function->fn_retinput));
 
 				/*
 				 * install $0 reference, but only for polymorphic return
@@ -803,7 +801,6 @@ plpgsql_compile_inline(char *proc_source)
 	char	   *func_name = "inline_code_block";
 	PLpgSQL_function *function;
 	ErrorContextCallback plerrcontext;
-	Oid			typinput;
 	PLpgSQL_variable *var;
 	int			parse_rc;
 	MemoryContext func_cxt;
@@ -876,8 +873,6 @@ plpgsql_compile_inline(char *proc_source)
 	/* a bit of hardwired knowledge about type VOID here */
 	function->fn_retbyval = true;
 	function->fn_rettyplen = sizeof(int32);
-	getTypeInputInfo(VOIDOID, &typinput, &function->fn_rettypioparam);
-	fmgr_info(typinput, &(function->fn_retinput));
 
 	/*
 	 * Remember if function is STABLE/IMMUTABLE.  XXX would it be better to
@@ -2200,12 +2195,11 @@ build_datatype(HeapTuple typeTup, int32 typmod, Oid collation)
 	}
 	typ->typlen = typeStruct->typlen;
 	typ->typbyval = typeStruct->typbyval;
+	typ->typtype = typeStruct->typtype;
 	typ->typrelid = typeStruct->typrelid;
-	typ->typioparam = getTypeIOParam(typeTup);
 	typ->collation = typeStruct->typcollation;
 	if (OidIsValid(collation) && OidIsValid(typ->collation))
 		typ->collation = collation;
-	fmgr_info(typeStruct->typinput, &(typ->typinput));
 	typ->atttypmod = typmod;
 
 	return typ;

src/pl/plpgsql/src/plpgsql.h

@@ -22,6 +22,7 @@
 #include "commands/event_trigger.h"
 #include "commands/trigger.h"
 #include "executor/spi.h"
+#include "utils/hsearch.h"
 
 /**********************************************************************
  * Definitions
@@ -178,10 +179,9 @@ typedef struct
 	int			ttype;			/* PLPGSQL_TTYPE_ code */
 	int16		typlen;			/* stuff copied from its pg_type entry */
 	bool		typbyval;
+	char		typtype;
 	Oid			typrelid;
-	Oid			typioparam;
 	Oid			collation;		/* from pg_type, but can be overridden */
-	FmgrInfo	typinput;		/* lookup info for typinput function */
 	int32		atttypmod;		/* typmod (taken from someplace else) */
 } PLpgSQL_type;
 
@@ -709,8 +709,6 @@ typedef struct PLpgSQL_function
 	Oid			fn_rettype;
 	int			fn_rettyplen;
 	bool		fn_retbyval;
-	FmgrInfo	fn_retinput;
-	Oid			fn_rettypioparam;
 	bool		fn_retistuple;
 	bool		fn_retset;
 	bool		fn_readonly;
@@ -748,6 +746,9 @@ typedef struct PLpgSQL_function
 	PLpgSQL_datum **datums;
 	PLpgSQL_stmt_block *action;
 
+	/* table for performing casts needed in this function */
+	HTAB	   *cast_hash;
+
 	/* these fields change when the function is used */
 	struct PLpgSQL_execstate *cur_estate;
 	unsigned long use_count;