The pg-el API

The entry points in the pg-el library are documented below.

(with-pg-connection con (dbname user [password host port]) &body body)

A macro which opens a TCP network connection to database DBNAME, executes the BODY forms then disconnects. See function pg-connect for details of the connection arguments.

(with-pg-connection-local con (path dbname user [password]) &body body)

A macro which opens a connection to database DBNAME over a local Unix socket at PATH, executes the BODY forms then disconnects. See function pg-connect-local for details of the connection arguments.

(with-pg-transaction con &body body)

A macro which executes the BODY forms wrapped in an SQL transaction. CON is a connection to the database. If an error occurs during the execution of the forms, a ROLLBACK instruction is executed.

(pg-connect dbname user [password host port tls-options]) -> con

Connect to the database DBNAME on HOST (defaults to localhost) at PORT (defaults to 5432) via TCP/IP and authenticate as USER with PASSWORD. This library currently supports SCRAM-SHA-256 authentication (the default method from PostgreSQL version 14 onwards), MD5 authentication and cleartext password authentication. This function also sets the output date type to ISO and initializes our type parser tables.

If tls-options is non-NIL, attempt to establish an encrypted connection to PostgreSQL by passing tls-options to Emacs function gnutls-negotiate. tls-options is a Common-Lisp style argument list of the form

(list :priority-string "NORMAL:-MD5" :trustfiles (list "/etc/company/RootCA.crt"))

To use client certificates to authenticate the TLS connection, use a value of TLS-OPTIONS of the form

`(list :keylist ((,key ,cert)))

where key is the filename of the client certificate private key and cert is the filename of the client certificate. These are passed to GnuTLS.

(pg-connect-local path dbname user [password]) -> con

Initiate a connection with the PostgreSQL backend over local Unix socket PATH. Connect to the database DBNAME with the username USER, providing PASSWORD if necessary. Returns a connection to the database (as an opaque type). PASSWORD defaults to an empty string.

(pg-exec con &rest sql) -> pgresult

Concatenate the SQL strings and send to the PostgreSQL backend over connection CON. Retrieve the information returned by the database and return it in an opaque record PGRESULT. The content of the pgresult should be accessed using the pg-result function.

(pg-exec-prepared con query typed-arguments &key (max-rows 0)) -> pgresult

Execute SQL query QUERY, which may include numbered parameters such as $1, $2 and so on, using PostgreSQL’s extended query protocol, on database connection CON. The TYPED-ARGUMENTS are a list of the form

'((42 . "int4") ("42" . "text"))

This query will return at most MAX-ROWS rows (a value of zero indicates no limit). It returns a pgresult structure (see function pg-result). This method is useful to reduce the risk of SQL injection attacks.

 (pg-result pgresult what &rest args) -> info

Extract information from the PGRESULT returned by pg-exec. The WHAT keyword can be one of

• :connection: retrieve the database connection.

• :status: a string returned by the backend to indicate the status of the command; it is something like “SELECT” for a select command, “DELETE 1” if the deletion affected a single row, etc.

• :attributes: a list of tuples providing metadata: the first component of each tuple is the attribute’s name as a string, the second an integer representing its PostgreSQL type, and the third an integer representing the size of that type.

• :tuples: all the data retrieved from the database, as a list of lists, each list corresponding to one row of data returned by the backend.

• :tuple tuple-number: return a specific tuple (numbering starts at 0).

• :incomplete: determine whether the set of tuples returned in this query set is incomplete, due to a suspended portal. If true, further tuples can be obtained by calling pg-fetch.

• :oid: allows you to retrieve the OID returned by the backend if the command was an insertion. The OID is a unique identifier for that row in the database (this is PostgreSQL-specific; please refer to the documentation for more details).

.

(pg-fetch con result &key (max-rows 0))

Fetch pending rows from the suspended portal in RESULT on database connection CON. This query will retrieve at most MAX-ROWS rows (default value of zero means no limit). Returns a pgresult structure (see function pg-result). When used in multiple fetch situations (with the :max-rows parameter to pg-exec-prepared which allows you to retrieve large result sets incrementally), the same pgresult structure (initally returned by pg-exec-prepared) should be passed to each successive call to pg-fetch, because it contains column metainformation that is required to parse the incoming data. Each successive call to pg-fetch will return this pgresult structure with new tuples accessible via pg-result :tuples. When no more tuples are available, the :incomplete slot of the pgresult structure will be nil.

(pg-cancel con) -> nil

Ask the server to cancel the command currently being processed by the backend. The cancellation request concerns the command requested over database connection CON.

(pg-disconnect con) -> nil

Close the database connection CON.

(pg-for-each con select-form callback)

Calls CALLBACK on each tuple returned by SELECT-FORM. Declares a cursor for SELECT-FORM, then fetches tuples using repeated executions of FETCH 1, until no results are left. The cursor is then closed. The work is performed within a transaction. When you have a large amount of data to handle, this usage is more efficient than fetching all the tuples in one go.

If you wish to browse the results, each one in a separate buffer, you could have the callback insert each tuple into a buffer created with (generate-new-buffer "myprefix"), then use ibuffer’s “/ n” to list/visit/delete all buffers whose names match myprefix.

(pg-databases con) -> list of strings

Return a list of the databases available over PostgreSQL connection CON. A database is a set of tables; in a fresh PostgreSQL installation there is a single database named “template1”.

(pg-tables con) -> list of strings

Return a list of the tables present in the database to which we are currently connected over CON. Only include user tables: system tables are not included in this list.

(pg-columns con table) -> list of strings

Return a list of the columns (or attributes) in TABLE, which must be a table in the database to which we are connected over CON. We only include the column names; if you want more detailed information (attribute types, for example), it can be obtained from pg-result on a SELECT statement for that table.

(pg-hstore-setup con)

Prepare for the use of HSTORE datatypes over database connection CON. This function must be called before using the HSTORE extension. It loads the extension if necessary, and sets up the parsing support for HSTORE datatypes.

(pg-vector-setup con)

Prepare for the use of VECTOR datatypes from the pgvector extension over database connection CON. This function must be called before using the pgvector extension. It loads the extension if necessary, and sets up the parsing support for vector datatypes.

(pg-lo-create con . args) -> oid

Create a new large object (BLOB, or binary large object in other DBMSes parlance) in the database to which we are connected via CON. Returns an OID (which is represented as an elisp integer) which will allow you to use the large object. Optional ARGS are a Unix-style mode string which determines the permissions of the newly created large object, one of “r” for read-only permission, “w” for write-only, “rw” for read+write. Default is “r”.

Large-object functions MUST be used within a transaction (see the macro with-pg-transaction).

(pg-lo-open con oid . args) -> fd

Open a large object whose unique identifier is OID (an elisp integer) in the database to which we are connected via CON. Optional ARGS is a Unix-style mode string as for pg-lo-create; which defaults to “r” read-only permissions. Returns a file descriptor (an elisp integer) which can be used in other large-object functions.

(pg-lo-close con fd)

Close the file descriptor FD which was associated with a large object. Note that this does not delete the large object; use pg-lo-unlink for that.

(pg-lo-read con fd bytes) -> string

Read BYTES from the file descriptor FD which is associated with a large object. Return an elisp string which should be BYTES characters long.

(pg-lo-write con fd buf)

Write the bytes contained in the elisp string BUF to the large object associated with the file descriptor FD.

(pg-lo-lseek con fd offset whence)

Do the equivalent of a lseek(2) on the file descriptor FD which is associated with a large object; i.e. reposition the read/write file offset for that large object to OFFSET (an elisp integer). WHENCE has the same significance as in lseek(); it should be one of SEEK_SET (set the offset to the absolute position), SEEK_CUR (set the offset relative to the current offset) or SEEK_END (set the offset relative to the end of the file). WHENCE should be an elisp integer whose values can be obtained from the header file <unistd.h> (probably 0, 1 and 2 respectively).

(pg-lo-tell con oid) -> integer

Do the equivalent of an ftell(3) on the file associated with the large object whose unique identifier is OID. Returns the current position of the file offset for the object’s associated file descriptor, as an elisp integer.

(pg-lo-unlink con oid)

Remove the large object whose unique identifier is OID from the system. In the current implementation of large objects in PostgreSQL, each large object is associated with an object in the filesystem.

(pg-lo-import con filename) -> oid

Create a new large object and initialize it to the data contained in the file whose name is FILENAME. Returns an OID (as an elisp integer). Note that this operation is only syntactic sugar around the basic large-object operations listed above.

(pg-lo-export con oid filename)

Create a new file named FILENAME and fill it with the contents of the large object whose unique identifier is OID. This operation is also syntactic sugar.

Variable pg-parameter-change-functions is a list of handlers to be called when the backend informs us of a parameter change, for example a change to the session time zone. Each handler is called with three arguments: the connection to the backend, the parameter name and the parameter value. It is initially set to a function that looks out for client_encoding messages and updates the value recorded in the connection.

Variable pg-handle-notice-functions is a list of handlers to be called when the backend sends us a NOTICE message. Each handler is called with one argument, the notice, as a pgerror struct.

Boolean variable pg-disable-type-coercion can be set to non-nil (before initiating a connection) to disable the library’s type coercion facility. Default is t.