On 2005-04-22, Shachar Shemesh <[EMAIL PROTECTED]> wrote: > Hi list, > > I'm trying to find out, from a client, how many characters will fit in a > varchar field > Problem is that when I do "PQfmod" on a varchar field defined as > "varchar(20)", PQfmod returns "24".
Interpreting PQfmod requires a rather intimate knowledge of the internal type implementations. For several types, including varchar, the typmod is rather arbitrarily the type's length limit plus the size of a varlena header (which appears to be 4 bytes on all platforms, even 64-bit ones). This is arbitrary because (thanks to multibyte characters) the value doesn't actually relate to the storage size; presumably it once did in the past. A quick summary of other types (accurate I believe as of 8.0): bit: typmod is the specified length exactly bpchar (i.e. char(n)): typmod is the length + VARHDRSZ numeric: this is ugly, the typmod is ((prec << 16) | scale) + VARHDRSZ, i.e. numeric(10,2) is ((10 << 16) | 2) + 4 interval: very complex due to attempts to support sql-standard intervals time, timestamp: typmod is the specified precision exactly varbit: typmod is the maximum bit length exactly varchar: typmod is the maximum length + VARHDRSZ -- Andrew, Supernews http://www.supernews.com - individual and corporate NNTP services ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
