Doc: document bpchar, clarify relationship of text and varchar.

For some reason the "bpchar" type name was defined nowhere in
our SGML docs, although several places refer to it in passing.
Give it a proper mention under Character Types.

While here, also provide an explanation of how the text and varchar
types relate.  The previous wording seemed to be doing its best
to sweep text under the rug, which doesn't seem very appropriate
given its prominence in other parts of the docs.

Minor rearrangements and word-smithing for clarity, too.

Laurenz Albe and Tom Lane, per gripe from Yanliang Lei

Discussion: https://postgr.es/m/120b3084.56b6.1833b5ffe4b[email protected]

diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 0258b192e0e4f18d3730efc45f841e78124d82a1..b030b36002f4ba3bfb8fc26c1ab18b9958de671a 100644 (file)
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -1154,6 +1154,10 @@ SELECT '52093.89'::money::numeric::float8;
     varchar
    
 
+   
+    bpchar
+   
+
     
      Character Types
      
@@ -1169,7 +1173,7 @@ SELECT '52093.89'::money::numeric::float8;
         variable-length with limit
        
        
-        character(n), char(n)
+        character(n), char(n), bpchar(n)
         fixed-length, blank padded
        
        
@@ -1196,7 +1200,14 @@ SELECT '52093.89'::money::numeric::float8;
     error, unless the excess characters are all spaces, in which case
     the string will be truncated to the maximum length. (This somewhat
     bizarre exception is required by the SQL
-    standard.) If the string to be stored is shorter than the declared
+    standard.)
+    However, if one explicitly casts a value to character
+    varying(n) or
+    character(n), then an over-length
+    value will be truncated to n characters without
+    raising an error. (This too is required by the
+    SQL standard.)
+    If the string to be stored is shorter than the declared
     length, values of type character will be space-padded;
     values of type character varying will simply store the
     shorter
@@ -1204,33 +1215,35 @@ SELECT '52093.89'::money::numeric::float8;
    
 
    
-    If one explicitly casts a value to character
-    varying(n) or
-    character(n), then an over-length
-    value will be truncated to n characters without
-    raising an error. (This too is required by the
-    SQL standard.)
+    In addition, PostgreSQL provides the
+    text type, which stores strings of any length.
+    Although the text type is not in the
+    SQL standard, several other SQL database
+    management systems have it as well.
+    text is PostgreSQL's native
+    string data type, in that most built-in functions operating on strings
+    are declared to take or return text not character
+    varying.  For many purposes, character varying
+    acts as though it were a domain
+    over text.
    
 
    
-    The notations varchar(n) and
-    char(n) are aliases for character
-    varying(n) and
-    character(n), respectively.
-    If specified, the length must be greater than zero and cannot exceed
-    10485760.
-    character without length specifier is equivalent to
-    character(1). If character varying is used
-    without length specifier, the type accepts strings of any size. The
-    latter is a PostgreSQL extension.
+    The type name varchar is an alias for character
+    varying, while char and bpchar are
+    aliases for character.
+    The varchar and char aliases are defined in
+    the SQL standard, but bpchar is
+    a PostgreSQL extension.
    
 
    
-    In addition, PostgreSQL provides the
-    text type, which stores strings of any length.
-    Although the type text is not in the
-    SQL standard, several other SQL database
-    management systems have it as well.
+    If specified, the length n must be greater
+    than zero and cannot exceed 10485760.
+    character without length specifier is equivalent to
+    character(1). If character varying is used
+    without length specifier, the type accepts strings of any size. The
+    latter behavior is a PostgreSQL extension.