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1.3. Command Execution Functions

Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.

1.3.1. Main Routines

  • PQexec Submit a command to the server and wait for the result.

    PGresult *PQexec(PGconn *conn,
                     const char *query);
    

    Returns a PGresult pointer or possibly a NULL pointer. A non-NULL pointer will generally be returned except in out-of-memory conditions or serious errors such as inability to send the command to the backend. If a NULL is returned, it should be treated like a PGRES_FATAL_ERROR result. Use PQerrorMessage to get more information about the error.

The PGresult structure encapsulates the result returned by the backend. libpq application programmers should be careful to maintain the PGresult abstraction. Use the accessor functions below to get at the contents of PGresult. Avoid directly referencing the fields of the PGresult structure because they are subject to change in the future. (Beginning in PostgreSQL 6.4, the definition of struct PGresult is not even provided in libpq-fe.h. If you have old code that accesses PGresult fields directly, you can keep using it by including libpq-int.h too, but you are encouraged to fix the code soon.)

  • PQresultStatus Returns the result status of the command.

    ExecStatusType PQresultStatus(const PGresult *res)
    

    PQresultStatus can return one of the following values:

    • PGRES_EMPTY_QUERY -- The string sent to the backend was empty.

    • PGRES_COMMAND_OK -- Successful completion of a command returning no data

    • PGRES_TUPLES_OK -- The query successfully executed

    • PGRES_COPY_OUT -- Copy Out (from server) data transfer started

    • PGRES_COPY_IN -- Copy In (to server) data transfer started

    • PGRES_BAD_RESPONSE -- The server's response was not understood

    • PGRES_NONFATAL_ERROR

    • PGRES_FATAL_ERROR

    If the result status is PGRES_TUPLES_OK, then the routines described below can be used to retrieve the rows returned by the query. Note that a SELECT command that happens to retrieve zero rows still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return rows (INSERT, UPDATE, etc.). A response of PGRES_EMPTY_QUERY often exposes a bug in the client software.

  • PQresStatus Converts the enumerated type returned by PQresultStatus into a string constant describing the status code.

    char *PQresStatus(ExecStatusType status);
    
  • PQresultErrorMessage returns the error message associated with the query, or an empty string if there was no error.

    char *PQresultErrorMessage(const PGresult *res);
    

    Immediately following a PQexec or PQgetResult call, PQerrorMessage (on the connection) will return the same string as PQresultErrorMessage (on the result). However, a PGresult will retain its error message until destroyed, whereas the connection's error message will change when subsequent operations are done. Use PQresultErrorMessage when you want to know the status associated with a particular PGresult; use PQerrorMessage when you want to know the status from the latest operation on the connection.

  • PQclear Frees the storage associated with the PGresult. Every query result should be freed via PQclear when it is no longer needed.

    void PQclear(PQresult *res);
    

    You can keep a PGresult object around for as long as you need it; it does not go away when you issue a new query, nor even if you close the connection. To get rid of it, you must call PQclear. Failure to do this will result in memory leaks in the frontend application.

  • PQmakeEmptyPGresult Constructs an empty PGresult object with the given status.

    PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
    

    This is libpq's internal routine to allocate and initialize an empty PGresult object. It is exported because some applications find it useful to generate result objects (particularly objects with error status) themselves. If conn is not NULL and status indicates an error, the connection's current error message is copied into the PGresult. Note that PQclear should eventually be called on the object, just as with a PGresult returned by libpq itself.

1.3.2. Escaping Strings for Inclusion in SQL Commands

PQescapeStringConn escapes a string for use within an SQL command. This is useful when inserting data values as literal constants in SQL commands. Certain characters (such as quotes and backslashes) must be escaped to prevent them from being interpreted specially by the SQL parser. PQescapeStringConn performs this operation.

Tip: It is especially important to do proper escaping when handling strings that were received from an untrustworthy source. Otherwise there is a security risk: you are vulnerable to "SQL injection" attacks wherein unwanted SQL commands are fed to your database.

size_t PQescapeStringConn (PGconn *conn,
                           char *to, const char *from, size_t length,
                           int *error);

PQescapeStringConn writes an escaped version of the from string to the to buffer, escaping special characters so that they cannot cause any harm, and adding a terminating zero byte. The single quotes that must surround PostgreSQL string literals are not included in the result string; they should be provided in the SQL command that the result is inserted into. The parameter from points to the first character of the string that is to be escaped, and the length parameter gives the number of bytes in this string. A terminating zero byte is not required, and should not be counted in length. (If a terminating zero byte is found before length bytes are processed, PQescapeStringConn stops at the zero; the behavior is thus rather like strncpy.) to shall point to a buffer that is able to hold at least one more byte than twice the value of length, otherwise the behavior is undefined. Behavior is likewise undefined if the to and from strings overlap.

If the error parameter is not NULL, then *error is set to zero on success, nonzero on error. Presently the only possible error conditions involve invalid multibyte encoding in the source string. The output string is still generated on error, but it can be expected that the server will reject it as malformed. On error, a suitable message is stored in the conn object, whether or not error is NULL.

PQescapeStringConn returns the number of bytes written to to, not including the terminating zero byte.

size_t PQescapeString (char *to, const char *from, size_t length);

PQescapeString is an older, deprecated version of PQescapeStringConn; the difference is that it does not take conn or error parameters. Because of this, it cannot adjust its behavior depending on the connection properties (such as character encoding) and therefore it may give the wrong results. Also, it has no way to report error conditions.

PQescapeString can be used safely in single-threaded client programs that work with only one PostgreSQL connection at a time (in this case it can find out what it needs to know "behind the scenes"). In other contexts it is a security hazard and should be avoided in favor of PQescapeStringConn.

1.3.3. Escaping Binary Strings for Inclusion in SQL Commands

PQescapeByteaConn

Escapes binary data for use within an SQL command with the type bytea.

unsigned char *PQescapeByteaConn(PGconn *conn,
                                 const unsigned char *from,
                                 size_t from_length,
                                 size_t *to_length);

Certain byte values must be escaped (but all byte values can be escaped) when used as part of a bytea literal in an SQL statement. In general, to escape a byte, it is converted into the three digit octal number equal to the octet value, and preceded by one or two backslashes. The single quote (') and backslash (\) characters have special alternative escape sequences. PQescapeByteaConn performs this operation, escaping only the minimally required bytes.

The from parameter points to the first byte of the string that is to be escaped, and the from_length parameter gives the number of bytes in this binary string. (A terminating zero byte is neither necessary nor counted.) The to_length parameter points to a variable that will hold the resultant escaped string length. This result string length includes the terminating zero byte of the result.

PQescapeByteaConn returns an escaped version of the from parameter binary string in memory allocated with malloc(). This memory must be freed using free() when the result is no longer needed. The return string has all special characters replaced so that they can be properly processed by the PostgreSQL string literal parser, and the bytea input function. A terminating zero byte is also added. The single quotes that must surround PostgreSQL string literals are not part of the result string.

On error, a NULL pointer is returned, and a suitable error message is stored in the conn object. Currently, the only possible error is insufficient memory for the result string.

PQescapeBytea

PQescapeBytea is an older, deprecated version of PQescapeByteaConn.

unsigned char *PQescapeBytea(unsigned char *from,
                             size_t from_length,
                             size_t *to_length);

The only difference from PQescapeByteaConn is that PQescapeBytea does not take a PGconn parameter. Because of this, it cannot adjust its behavior depending on the connection properties and therefore it may give the wrong results. Also, it has no way to return an error message on failure.

PQescapeBytea can be used safely in single-threaded client programs that work with only one PostgreSQL connection at a time (in this case it can find out what it needs to know "behind the scenes"). In other contexts it is a security hazard and should be avoided in favor of PQescapeByteaConn.

PQunescapeBytea

Converts a string representation of binary data into binary data --- the reverse of PQescapeBytea. This is needed when retrieving bytea data in text format, but not when retrieving it in binary format.

unsigned char *PQunescapeBytea(unsigned char *from, size_t *to_length);

The from parameter points to a string such as might be returned by PQgetvalue when applied to a bytea column. PQunescapeBytea converts this string representation into its binary representation. It returns a pointer to a buffer allocated with malloc(), or null on error, and puts the size of the buffer in to_length. The result must be freed using free() when it is no longer needed.

This conversion is not exactly the inverse of PQescapeBytea, because the string is not expected to be "escaped" when received from PQgetvalue. In particular this means there is no need for string quoting considerations, and so no need for a PGconn parameter.

1.3.4. Retrieving SELECT Result Information

  • PQntuples Returns the number of tuples (rows) in the query result.

    int PQntuples(const PGresult *res);
    
  • PQnfields Returns the number of fields (columns) in each row of the query result.

    int PQnfields(const PGresult *res);
    
  • PQfname Returns the field (column) name associated with the given field index. Field indices start at 0.

    char *PQfname(const PGresult *res,
                        int field_index);
    
  • PQfnumber Returns the field (column) index associated with the given field name.

    int PQfnumber(const PGresult *res,
                  const char *field_name);
    

    -1 is returned if the given name does not match any field.

  • PQftype Returns the field type associated with the given field index. The integer returned is an internal coding of the type. Field indices start at 0.

    Oid PQftype(const PGresult *res,
                int field_index);
    

    You can query the system table pg_type to obtain the name and properties of the various data types. The OIDs of the built-in data types are defined in src/include/catalog/pg_type.h in the source tree.

  • PQfmod Returns the type-specific modification data of the field associated with the given field index. Field indices start at 0.

    int PQfmod(const PGresult *res,
               int field_index);
    
  • PQfsize Returns the size in bytes of the field associated with the given field index. Field indices start at 0.

    int PQfsize(const PGresult *res,
                int field_index);
    

    PQfsize returns the space allocated for this field in a database tuple, in other words the size of the server's binary representation of the data type. -1 is returned if the field is variable size.

  • PQbinaryTuples Returns 1 if the PGresult contains binary tuple data, 0 if it contains ASCII data.

    int PQbinaryTuples(const PGresult *res);
    

    Currently, binary tuple data can only be returned by a query that extracts data from a binary cursor.

1.3.5. Retrieving SELECT Result Values

  • PQgetvalue Returns a single field (column) value of one tuple (row) of a PGresult. Tuple and field indices start at 0.

    char* PQgetvalue(const PGresult *res,
                     int tup_num,
                     int field_num);
    

    For most queries, the value returned by PQgetvalue is a null-terminated character string representation of the attribute value. But if PQbinaryTuples() is 1, the value returned by PQgetvalue is the binary representation of the type in the internal format of the backend server (but not including the size word, if the field is variable-length). It is then the programmer's responsibility to cast and convert the data to the correct C type. The pointer returned by PQgetvalue points to storage that is part of the PGresult structure. One should not modify it, and one must explicitly copy the value into other storage if it is to be used past the lifetime of the PGresult structure itself.

  • PQgetisnull Tests a field for a NULL entry. Tuple and field indices start at 0.

    int PQgetisnull(const PGresult *res,
                    int tup_num,
                    int field_num);
    

    This function returns 1 if the field contains a NULL, 0 if it contains a non-null value. (Note that PQgetvalue will return an empty string, not a null pointer, for a NULL field.)

  • PQgetlength Returns the length of a field (attribute) value in bytes. Tuple and field indices start at 0.

    int PQgetlength(const PGresult *res,
                    int tup_num,
                    int field_num);
    

    This is the actual data length for the particular data value, that is the size of the object pointed to by PQgetvalue. Note that for character-represented values, this size has little to do with the binary size reported by PQfsize.

  • PQprint Prints out all the tuples and, optionally, the attribute names to the specified output stream.

    void PQprint(FILE* fout,      /* output stream */
                 const PGresult *res,
                 const PQprintOpt *po);
    
    struct {
        pqbool  header;      /* print output field headings and row count */
        pqbool  align;       /* fill align the fields */
        pqbool  standard;    /* old brain dead format */
        pqbool  html3;       /* output html tables */
        pqbool  expanded;    /* expand tables */
        pqbool  pager;       /* use pager for output if needed */
        char    *fieldSep;   /* field separator */
        char    *tableOpt;   /* insert to HTML table ... */
        char    *caption;    /* HTML caption */
        char    **fieldName; /* null terminated array of replacement field names */
    } PQprintOpt;
           
    

    This function was formerly used by psql to print query results, but this is no longer the case and this function is no longer actively supported.

1.3.6. Retrieving Non-SELECT Result Information

  • PQcmdStatus Returns the command status string from the SQL command that generated the PGresult.

    char * PQcmdStatus(PGresult *res);
    
  • PQcmdTuples Returns the number of rows affected by the SQL command.

    char * PQcmdTuples(PGresult *res);
    

    If the SQL command that generated the PGresult was INSERT, UPDATE or DELETE, this returns a string containing the number of rows affected. If the command was anything else, it returns the empty string.

  • PQoidValue Returns the object ID of the inserted row, if the SQL command was an INSERT that inserted exactly one row into a table that has OIDs. Otherwise, returns InvalidOid.

    Oid PQoidValue(const PGresult *res);
    

    The type Oid and the constant InvalidOid will be defined if you include the libpq header file. They will both be some integer type.

  • PQoidStatus Returns a string with the object ID of the inserted row, if the SQL command was an INSERT. (The string will be 0 if the INSERT did not insert exactly one row, or if the target table does not have OIDs.) If the command was not an INSERT, returns an empty string.

    char * PQoidStatus(const PGresult *res);
    

    This function is deprecated in favor of PQoidValue and is not thread-safe.

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