COPY
CommandThe COPY
command in PostgreSQL has options to read from or write to the network connection used by libpq. The functions described in this section allow applications to take advantage of this capability by supplying or consuming copied data.
The overall process is that the application first issues the SQL COPY
command via PQexec
or one of the equivalent functions. The response to this (if there is no error in the command) will be a PGresult
object bearing a status code of PGRES_COPY_OUT
or PGRES_COPY_IN
(depending on the specified copy direction). The application should then use the functions of this section to receive or transmit data rows. When the data transfer is complete, another PGresult
object is returned to indicate success or failure of the transfer. Its status will be PGRES_COMMAND_OK
for success or PGRES_FATAL_ERROR
if some problem was encountered. At this point further SQL commands can be issued via PQexec
. (It is not possible to execute other SQL commands using the same connection while the COPY
operation is in progress.)
If a COPY
command is issued via PQexec
in a string that could contain additional commands, the application must continue fetching results via PQgetResult
after completing the COPY
sequence. Only when PQgetResult
returns NULL
is it certain that the PQexec
command string is done and it is safe to issue more commands.
The functions of this section should be executed only after obtaining a result status of PGRES_COPY_OUT
or PGRES_COPY_IN
from PQexec
or PQgetResult
.
A PGresult
object bearing one of these status values carries some additional data about the COPY
operation that is starting. This additional data is available using functions that are also used in connection with query results:
PQnfields
Returns the number of columns (fields) to be copied.
PQbinaryTuples
0 indicates the overall copy format is textual (rows separated by newlines, columns separated by separator characters, etc). 1 indicates the overall copy format is binary. See COPY for more information.
PQfformat
Returns the format code (0 for text, 1 for binary) associated with each column of the copy operation. The per-column format codes will always be zero when the overall copy format is textual, but the binary format can support both text and binary columns. (However, as of the current implementation of COPY
, only binary columns appear in a binary copy; so the per-column formats always match the overall format at present.)
These additional data values are only available when using protocol 3.0. When using protocol 2.0, all these functions will return 0.
COPY
DataThese functions are used to send data during COPY FROM STDIN
. They will fail if called when the connection is not in COPY_IN
state.
PQputCopyData
Sends data to the server during COPY_IN
state.
int PQputCopyData(PGconn *conn, const char *buffer, int nbytes);
Transmits the COPY
data in the specified buffer
, of length nbytes
, to the server. The result is 1 if the data was queued, zero if it was not queued because of full buffers (this will only happen in nonblocking mode), or -1 if an error occurred. (Use PQerrorMessage
to retrieve details if the return value is -1. If the value is zero, wait for write-ready and try again.)
The application can divide the COPY
data stream into buffer loads of any convenient size. Buffer-load boundaries have no semantic significance when sending. The contents of the data stream must match the data format expected by the COPY
command; see COPY for details.
PQputCopyEnd
Sends end-of-data indication to the server during COPY_IN
state.
int PQputCopyEnd(PGconn *conn, const char *errormsg);
Ends the COPY_IN
operation successfully if errormsg
is NULL
. If errormsg
is not NULL
then the COPY
is forced to fail, with the string pointed to by errormsg
used as the error message. (One should not assume that this exact error message will come back from the server, however, as the server might have already failed the COPY
for its own reasons. Also note that the option to force failure does not work when using pre-3.0-protocol connections.)
The result is 1 if the termination message was sent; or in nonblocking mode, this may only indicate that the termination message was successfully queued. (In nonblocking mode, to be certain that the data has been sent, you should next wait for write-ready and call PQflush
, repeating until it returns zero.) Zero indicates that the function could not queue the termination message because of full buffers; this will only happen in nonblocking mode. (In this case, wait for write-ready and try the PQputCopyEnd
call again.) If a hard error occurs, -1 is returned; you can use PQerrorMessage
to retrieve details.
After successfully calling PQputCopyEnd
, call PQgetResult
to obtain the final result status of the COPY
command. One can wait for this result to be available in the usual way. Then return to normal operation.
COPY
DataThese functions are used to receive data during COPY TO STDOUT
. They will fail if called when the connection is not in COPY_OUT
state.
PQgetCopyData
Receives data from the server during COPY_OUT
state.
int PQgetCopyData(PGconn *conn, char **buffer, int async);
Attempts to obtain another row of data from the server during a COPY
. Data is always returned one data row at a time; if only a partial row is available, it is not returned. Successful return of a data row involves allocating a chunk of memory to hold the data. The buffer
parameter must be non-NULL
. *buffer
is set to point to the allocated memory, or to NULL
in cases where no buffer is returned. A non-NULL
result buffer should be freed using PQfreemem
when no longer needed.
When a row is successfully returned, the return value is the number of data bytes in the row (this will always be greater than zero). The returned string is always null-terminated, though this is probably only useful for textual COPY
. A result of zero indicates that the COPY
is still in progress, but no row is yet available (this is only possible when async
is true). A result of -1 indicates that the COPY
is done. A result of -2 indicates that an error occurred (consult PQerrorMessage
for the reason).
When async
is true (not zero), PQgetCopyData
will not block waiting for input; it will return zero if the COPY
is still in progress but no complete row is available. (In this case wait for read-ready and then call PQconsumeInput
before calling PQgetCopyData
again.) When async
is false (zero), PQgetCopyData
will block until data is available or the operation completes.
After PQgetCopyData
returns -1, call PQgetResult
to obtain the final result status of the COPY
command. One can wait for this result to be available in the usual way. Then return to normal operation.
COPY
These functions represent older methods of handling COPY
. Although they still work, they are deprecated due to poor error handling, inconvenient methods of detecting end-of-data, and lack of support for binary or nonblocking transfers.
PQgetline
Reads a newline-terminated line of characters (transmitted by the server) into a buffer string of size length
.
int PQgetline(PGconn *conn, char *buffer, int length);
This function copies up to length
-1 characters into the buffer and converts the terminating newline into a zero byte. PQgetline
returns EOF
at the end of input, 0 if the entire line has been read, and 1 if the buffer is full but the terminating newline has not yet been read.
Note that the application must check to see if a new line consists of the two characters \.
, which indicates that the server has finished sending the results of the COPY
command. If the application might receive lines that are more than length
-1 characters long, care is needed to be sure it recognizes the \.
line correctly (and does not, for example, mistake the end of a long data line for a terminator line).
PQgetlineAsync
Reads a row of COPY
data (transmitted by the server) into a buffer without blocking.
int PQgetlineAsync(PGconn *conn, char *buffer, int bufsize);
This function is similar to PQgetline
, but it can be used by applications that must read COPY
data asynchronously, that is, without blocking. Having issued the COPY
command and gotten a PGRES_COPY_OUT
response, the application should call PQconsumeInput
and PQgetlineAsync
until the end-of-data signal is detected.
Unlike PQgetline
, this function takes responsibility for detecting end-of-data.
On each call, PQgetlineAsync
will return data if a complete data row is available in libpq's input buffer. Otherwise, no data is returned until the rest of the row arrives. The function returns -1 if the end-of-copy-data marker has been recognized, or 0 if no data is available, or a positive number giving the number of bytes of data returned. If -1 is returned, the caller must next call PQendcopy
, and then return to normal processing.
The data returned will not extend beyond a data-row boundary. If possible a whole row will be returned at one time. But if the buffer offered by the caller is too small to hold a row sent by the server, then a partial data row will be returned. With textual data this can be detected by testing whether the last returned byte is \n
or not. (In a binary COPY
, actual parsing of the COPY
data format will be needed to make the equivalent determination.) The returned string is not null-terminated. (If you want to add a terminating null, be sure to pass a bufsize
one smaller than the room actually available.)
PQputline
Sends a null-terminated string to the server. Returns 0 if OK and EOF
if unable to send the string.
int PQputline(PGconn *conn, const char *string);
The COPY
data stream sent by a series of calls to PQputline
has the same format as that returned by PQgetlineAsync
, except that applications are not obliged to send exactly one data row per PQputline
call; it is okay to send a partial line or multiple lines per call.
Before PostgreSQL protocol 3.0, it was necessary for the application to explicitly send the two characters \.
as a final line to indicate to the server that it had finished sending COPY
data. While this still works, it is deprecated and the special meaning of \.
can be expected to be removed in a future release. It is sufficient to call PQendcopy
after having sent the actual data.
PQputnbytes
Sends a non-null-terminated string to the server. Returns 0 if OK and EOF
if unable to send the string.
int PQputnbytes(PGconn *conn, const char *buffer, int nbytes);
This is exactly like PQputline
, except that the data buffer need not be null-terminated since the number of bytes to send is specified directly. Use this procedure when sending binary data.
PQendcopy
Synchronizes with the server.
int PQendcopy(PGconn *conn);
This function waits until the server has finished the copying. It should either be issued when the last string has been sent to the server using PQputline
or when the last string has been received from the server using PQgetline
. It must be issued or the server will get “out of sync” with the client. Upon return from this function, the server is ready to receive the next SQL command. The return value is 0 on successful completion, nonzero otherwise. (Use PQerrorMessage
to retrieve details if the return value is nonzero.)
When using PQgetResult
, the application should respond to a PGRES_COPY_OUT
result by executing PQgetline
repeatedly, followed by PQendcopy
after the terminator line is seen. It should then return to the PQgetResult
loop until PQgetResult
returns a null pointer. Similarly a PGRES_COPY_IN
result is processed by a series of PQputline
calls followed by PQendcopy
, then return to the PQgetResult
loop. This arrangement will ensure that a COPY
command embedded in a series of SQL commands will be executed correctly.
Older applications are likely to submit a COPY
via PQexec
and assume that the transaction is done after PQendcopy
. This will work correctly only if the COPY
is the only SQL command in the command string.