如何为数据库对象名称使用postgres dbd占位符

Question

(or How to iterate thru information schema Using perl DBI (DBD::PG) and placeholders?)

Windows 7, ActiveState Perl 5.20.2, PostgreSQL 9.4.1 .

Cases A, B and C below were successful when using a placeholder for a COLUMN VALUE. In order

• no placeholder used

• passed a literal

• passed a variable (populated with same literal)

It would be great to raise it up a level to DB Objects.. (tables, views etc)

Here's the output with the error for Case D:

Z:\CTAM\data_threat_mapping\DB Stats\perl scripts>test_placeholder.pl

A Row Count: 1
B Row Count: 1
C Row Count: 1

DBD::Pg::st execute failed: ERROR:  syntax error at or near "$1"

LINE 1: SELECT COUNT(*) FROM $1 WHERE status = 'Draft';
                             ^ at Z:\CTAM\data_threat_mapping\DB     Stats\perl 
scripts\test_placeholder.pl line 34.

Much obliged for any direction!

#!/usr/bin/perl -w
use strict;
use diagnostics;
use DBI;

my $num_rows = 0;

# connect
my $dbh = DBI->connect("DBI:Pg:dbname=CTAM;host=localhost",
                       "postgres", "xxxxx",
                       { 'RaiseError' => 1, pg_server_prepare => 1 });

#---------------------
# A - success
my $sthA = $dbh->prepare(
    "SELECT COUNT(*) FROM cwe_compound_element WHERE status = 'Draft';"
);
$sthA->execute(); # no placeholders

#---------------------
# B -  success
my $sthB = $dbh->prepare (
    "SELECT COUNT(*) FROM cwe_compound_element WHERE status = ?;"
);
$sthB->execute('Draft'); # pass 'Draft' to placeholder

#---------------------
# C -  success
my $status_value = 'Draft';
my $sthC = $dbh->prepare(
    "SELECT COUNT(*) FROM cwe_compound_element WHERE status = ?;"
);
$sthC->execute($status_value); # pass variable (column value) to placeholder

#---------------------
# D - failure
my $sthD = $dbh->prepare(
    "SELECT COUNT(*) FROM ? WHERE status = 'Draft';"
);
$sthD->execute('cwe_compound_element'); # pass tablename to placeholder

I've tried single/double/sans quotes (q, qq)...

Answer 1

If

SELECT * FROM Foo WHERE field = ?

means

SELECT * FROM Foo WHERE field = 'val'

then

SELECT * FROM ?

means

SELECT * FROM 'Table'

and that's obviously wrong. Placeholders can only be used in an expression. Fix:

my $sthD = $dbh->prepare("
   SELECT COUNT(*)
    FROM ".$dbh->quote_identifier($table)."
   WHERE status = 'Draft'
");
$sthD->execute();