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      does not exist" errors when accessing temporary tables in PL/PgSQL
      functions?<BR>
       <A href="#4.20">4.20</A>) What replication solutions are available?<BR>
!      
  
      <HR>
  
--- 116,124 ----
      does not exist" errors when accessing temporary tables in PL/PgSQL
      functions?<BR>
       <A href="#4.20">4.20</A>) What replication solutions are available?<BR>
!      <A href="#4.21">4.21</A>) Why are my table and column names not
!     recognized in my query?<BR>
! 
  
      <HR>
  
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      <P>The maximum table size and maximum number of columns can be
      quadrupled by increasing the default block size to 32k.</P>
  
+     <P>One limitation is that indexes can not be created on columns
+     longer than about 2,000 characters. Fortunately, such indexes are
+     rarely needed. Uniqueness is best guaranteed using another column
+     that is an MD5 hash of the long column, and full text indexing
+     allows for searching of words within the column.</P>
+ 
      <H3><A name="4.5">4.5</A>) How much database disk space is required
      to store data from a typical text file?</H3>
  
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  <PRE>
      CREATE INDEX tabindex ON tab (lower(col));
  </PRE>
! 
      <H3><A name="4.9">4.9</A>) In a query, how do I detect if a field
      is <SMALL>NULL</SMALL>?  How can I sort on whether a field is <SMALL>
      NULL</SMALL> or not?</H3>
--- 750,761 ----
  <PRE>
      CREATE INDEX tabindex ON tab (lower(col));
  </PRE>
!     <P>If the above index is created as <SMALL>UNIQUE</SMALL>, though
!     the column can store upper and lowercase characters, it can not have
!     identical values that differ only in case. To force a particular
!     case to be stored in the column, use a <SMALL>CHECK</SMALL>
!     constraint or a trigger.</P>
! 					 
      <H3><A name="4.9">4.9</A>) In a query, how do I detect if a field
      is <SMALL>NULL</SMALL>?  How can I sort on whether a field is <SMALL>
      NULL</SMALL> or not?</H3>
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      <P>There are also commercial and hardware-based replication solutions
      available supporting a variety of replication models.</P>
+ 
+     <H3><A name="4.20">4.20</A>) Why are my table and column names not
+     recognized in my query?</H3>
+ 
+     <P>The most common cause is the use of double-quotes around table or
+     column names during table creation. When double-quotes are used,
+     table and column names (called identifiers) are stored <a
+     href="http://www.postgresql.org/docs/8.0/static/sql-syntax.html#SQL-
+     SYNTAX-IDENTIFIERS">case-sensitive</a>, meaning you must use
+     double-quotes when referencing the names in a query. Some interfaces,
+     like pgAdmin, automatically double-quote identifiers during table
+     creation. So, for identifiers to be recognized, you must either:
+     <UL>
+     <LI>Avoid double-quoting identifiers when creating tables</LI>
+     <LI>Use only lowercase characters in identifiers</LI>
+     <LI>Double-quote identifiers when referencing them in queries</LI>
+     </UL>
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