And could someone tell why the following code does not work? It does not work either with or without placeholders, but with different errors.

In this case, I want to select all unique dates from timestamps. It works as expected with select distinct cast(date as date) from table; issued directly in psql.

# With placeholders
my $sth_test = $dbh->prepare("select distinct cast($date_col as date) 
+from ?;") or die;
$sth_test->execute($schema.$table);

# Errors (on execute())
INFO: The database has been opened successfully.
DBD::Pg::st execute failed: ERROR:  syntax error at or near "$1"
LINE 1: select distinct cast(date as date) from $1;
                                                       ^ at ./refresh_
+dates.pl line n.
DBD::Pg::st execute failed: ERROR:  syntax error at or near "$1"
LINE 1: select distinct cast(date as date) from $1;
                                                       ^ at ./refresh_
+dates.pl line n.


# Without placeholders
my $sth_test = $dbh->prepare("select distinct cast($date_col as date) 
+from $schema.$table;") or die;
$sth_test->execute();
$dbh->disconnect();

# Error (on disconnect)
DBI::db=HASH(0x32cec98)->disconnect invalidates 1 active statement han
+dle (either destroy statement handles or call finish on them before d
+isconnecting) at ./refresh_dates.pl line n.
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Placeholders cannot be used for table names or schema names or anything within the schema. They may only be used for data.

The problem was that the statement handle was not finished. Solution is to add $sth_test->finish(); before disconnecting the database (src).<\p>

However, I never used this command before and never encountered this error. Should I use it always before disconnecting the database?