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Add notion of a "transform function" that can simplify function calls.

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Initially, we use this only to eliminate calls to the varchar()
function in cases where the length is not being reduced and, therefore,
the function call is equivalent to a RelabelType operation.  The most
significant effect of this is that we can avoid a table rewrite when
changing a varchar(X) column to a varchar(Y) column, where Y > X.

Noah Misch, reviewed by me and Alexey Klyukin
This commit is contained in:
Robert Haas
2011-06-21 22:15:24 -04:00
parent 771a9f69f7
commit 8f9fe6edce
11 changed files with 2401 additions and 2280 deletions
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34
src/backend/utils/adt/varchar.c
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@@ -18,6 +18,8 @@
#include "access/hash.h"
#include "access/tuptoaster.h"
#include "libpq/pqformat.h"
#include "nodes/nodeFuncs.h"
#include "parser/parse_clause.h"
#include "utils/array.h"
#include "utils/builtins.h"
#include "mb/pg_wchar.h"
@@ -548,6 +550,38 @@ varcharsend(PG_FUNCTION_ARGS)
}
/*
* Flatten calls to our length coercion function that leave the new maximum
* length >= the previous maximum length. We ignore the isExplicit argument,
* which only affects truncation.
*/
Datum
varchar_transform(PG_FUNCTION_ARGS)
{
FuncExpr *expr = (FuncExpr *) PG_GETARG_POINTER(0);
Node *typmod;
Node *ret = NULL;
if (!IsA(expr, FuncExpr))
PG_RETURN_POINTER(ret);
Assert(list_length(expr->args) == 3);
typmod = lsecond(expr->args);
if (IsA(typmod, Const))
{
Node *source = linitial(expr->args);
int32 new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
int32 old_max = exprTypmod(source) - VARHDRSZ;
int32 new_max = new_typmod - VARHDRSZ;
if (new_max < 0 || (old_max >= 0 && old_max <= new_max))
ret = relabel_to_typmod(source, new_typmod);
}
PG_RETURN_POINTER(ret);
}
/*
* Converts a VARCHAR type to the specified size.
*