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Add documentation and tests for quote marks in ECPG literal queries.

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ECPG's PREPARE ... FROM and EXECUTE IMMEDIATE can optionally take
the target query as a simple literal, rather than the more usual
string-variable reference.  This was previously documented as
being a C string literal, but that's a lie in one critical respect:
you can't write a data double quote as \" in such literals.  That's
because the lexer is in SQL mode at this point, so it'll parse
double-quoted strings as SQL identifiers, within which backslash
is not special, so \" ends the literal.

I looked into making this work as documented, but getting the lexer
to switch behaviors at just the right point is somewhere between
very difficult and impossible.  It's not really worth the trouble,
because these cases are next to useless: if you have a fixed SQL
statement to execute or prepare, you might as well write it as
a direct EXEC SQL, saving the messiness of converting it into
a string literal and gaining the opportunity for compile-time
SQL syntax checking.

Instead, let's just document (and test) the workaround of writing
a double quote as an octal escape (\042) in such cases.

There's no code behavioral change here, so in principle this could
be back-patched, but it's such a niche case I doubt it's worth
the trouble.

Per report from 1250kv.

Discussion: https://postgr.es/m/673825.1603223178@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2020-10-22 18:29:40 -04:00
parent 3dfb1942d9
commit c16a1bbcf4
5 changed files with 67 additions and 11 deletions
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59
doc/src/sgml/ecpg.sgml
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@ -7066,7 +7066,7 @@ EXECUTE IMMEDIATE <replaceable class="parameter">string</replaceable>
<term><replaceable class="parameter">string</replaceable></term>
<listitem>
<para>
A literal C string or a host variable containing the SQL
A literal string or a host variable containing the SQL
statement to be executed.
</para>
</listitem>
@ -7074,6 +7074,30 @@ EXECUTE IMMEDIATE <replaceable class="parameter">string</replaceable>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
In typical usage, the <replaceable>string</replaceable> is a host
variable reference to a string containing a dynamically-constructed
SQL statement. The case of a literal string is not very useful;
you might as well just write the SQL statement directly, without
the extra typing of <command>EXECUTE IMMEDIATE</command>.
</para>
<para>
If you do use a literal string, keep in mind that any double quotes
you might wish to include in the SQL statement must be written as
octal escapes (<literal>\042</literal>) not the usual C
idiom <literal>\"</literal>. This is because the string is inside
an <literal>EXEC SQL</literal> section, so the ECPG lexer parses it
according to SQL rules not C rules. Any embedded backslashes will
later be handled according to C rules; but <literal>\"</literal>
causes an immediate syntax error because it is seen as ending the
literal.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
@ -7388,7 +7412,7 @@ EXEC SQL OPEN :curname1;
<refsynopsisdiv>
<synopsis>
PREPARE <replaceable class="parameter">name</replaceable> FROM <replaceable class="parameter">string</replaceable>
PREPARE <replaceable class="parameter">prepared_name</replaceable> FROM <replaceable class="parameter">string</replaceable>
</synopsis>
</refsynopsisdiv>
@ -7421,15 +7445,40 @@ PREPARE <replaceable class="parameter">name</replaceable> FROM <replaceable clas
<term><replaceable class="parameter">string</replaceable></term>
<listitem>
<para>
A literal C string or a host variable containing a preparable
statement, one of the SELECT, INSERT, UPDATE, or
DELETE.
A literal string or a host variable containing a preparable
SQL statement, one of SELECT, INSERT, UPDATE, or DELETE.
Use question marks (<literal>?</literal>) for parameter values
to be supplied at execution.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
In typical usage, the <replaceable>string</replaceable> is a host
variable reference to a string containing a dynamically-constructed
SQL statement. The case of a literal string is not very useful;
you might as well just write a direct SQL <command>PREPARE</command>
statement.
</para>
<para>
If you do use a literal string, keep in mind that any double quotes
you might wish to include in the SQL statement must be written as
octal escapes (<literal>\042</literal>) not the usual C
idiom <literal>\"</literal>. This is because the string is inside
an <literal>EXEC SQL</literal> section, so the ECPG lexer parses it
according to SQL rules not C rules. Any embedded backslashes will
later be handled according to C rules; but <literal>\"</literal>
causes an immediate syntax error because it is seen as ending the
literal.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<programlisting>