MIME-Version: 1.0
From: Ashutosh Sharma <ashu.coek88@gmail.com>
Date: Tue, 24 Jul 2018 16:16:57 +0530
Message-ID: 
 <CAE9k0PmTBOFAjOzc814jWW_AcnyUqA4dLX245eLxAP3zHCirYQ@mail.gmail.com>
Subject: Host variables in ecpg
To: pgsql-interfaces@lists.postgresql.org
Cc: Michael Meskes <meskes@postgresql.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Hi All,

While trying to understand the concept of host variables in ecpg, I
came to know that the host variables in ecpg can be declared using the
following two methods,

1) Using the explicit declare section.

EXEC SQL BEGIN DECLARE SECTION;
double d = 3.14;
EXEC SQL END DECLARE SECTION;

2) Using an implicit declare section.

EXEC SQL double d = 3.14;


Incase of 1 (i.e. when using explicit declare section) the host
variables gets scanned in C state using '<C>{identifier}' rule whereas
incase of 2 (i.e. when using implicit declare section) the host
variables gets scanned in SQL state using '<SQL>{identifier}' rule. I
could observe that, in sql state, we perform SQL keyword lookup
followed by C keyword lookup. However, in case of C state, we just
perform C keyword lookup. Please refer to the following code snippets
in pgc.l file

<C>{identifier}     {
                        const ScanKeyword       *keyword;

                        if ((!INFORMIX_MODE || !isinformixdefine()) &&
!isdefine())
                        {
                            keyword = ScanCKeywordLookup(yytext);
                            if (keyword != NULL)
                                return keyword->value;
                            else
                            {
                                base_yylval.str = mm_strdup(yytext);
                                return IDENT;
                            }
                        }


<SQL>{identifier}   {
                        const ScanKeyword  *keyword;

                        if (!isdefine())
                        {
                            /* Is it an SQL/ECPG keyword? */
                            keyword = ScanECPGKeywordLookup(yytext);
                            if (keyword != NULL)
                                return keyword->value;

                            /* Is it a C keyword? */
                            keyword = ScanCKeywordLookup(yytext);
                            if (keyword != NULL)
                                return keyword->value;

                            /*
                             * None of the above.  Return it as an identifier.
                             *
                             * The backend will attempt to truncate
and case-fold
                             * the identifier, but I see no good reason for ecpg
                             * to do so; that's just another way that
ecpg could get
                             * out of step with the backend.
                             */
                            base_yylval.str = mm_strdup(yytext);
                            return IDENT;

Shouldn't we perform both SQL and C table lookup in C state as well?

Let's consider the following test where a double datatype is used
inside the declare section.

    EXEC SQL BEGIN DECLARE SECTION;
        double d1;
    EXEC SQL END DECLARE SECTION;

In above case "double" would be scanned in C state by <C>{identifiers}
rule which would search for double keyword in ScanCKeywords[] table
and since it doesn't find it, it returns some IDENT keyword for it.

However, if above is written as "EXEC SQL double d;" then, double
would be scanned in SQL state by <SQL>{identifiers} rule which would
first search for double in SQLScanKeywords[] followed by
ScanCKeywords[]. In this case, as double is declared in
SQLScanKeywords[], it would return some valid keyword for double.

--
With Regards,
Ashutosh Sharma
EnterpriseDB:http://www.enterprisedb.com