CL-postgres reference manual

The CL-postgres module implements a rather low-level interface for communicating with a PostgreSQL database server. It is part of the Postmodern library, but can be used separately.


  1. Connecting
  2. Querying
  3. Reading values
  4. Row readers
  5. Conditions
  6. Symbol-index


class database-connection

Objects of this type represent database connections.

function open-database (database user password host &optional (port 5432) (use-ssl :no))
→ database-connection

Create and open a connection for the specified server, database, and user. use-ssl may be :no, :yes, or :try, where :try means 'if the server supports it'. When it is anything but :no, you must have the CL+SSL package loaded to initiate the connection.

On SBCL, the value :unix may be passed for host, in order to connect using a Unix domain socket instead of a TCP socket.

function close-database (database-connection)

Close a database connection. It is advisable to call this on connections when you are done with them. Otherwise the open socket will stick around until it is garbage collected, and no one will tell the database server that we are done with it.

function reopen-database (database-connection)

Re-establish a database connection for a previously closed connection object. (Calling this on a connection that is still open is harmless.)

function database-open-p (database-connection)
→ boolean

Test whether a database connection is still open.

method connection-meta (database-connection)
→ hash-table

This method provides access to a hash table that is associated with the current database connection. When the connection is closed and re-opened this hash table is reset. The most obvious use for this is for storing information about the prepared statements that have been parsed for this connection.

method connection-parameters (database-connection)
→ hash-table

This method returns a mapping (string to string) containing all the configuration parameters for the connection.

variable *ssl-certificate-file*
variable *ssl-key-file*

When using SSL (see open-database), these can be used to provide client key and certificate files. They can be either NIL, for no file, or a pathname.


function exec-query (database-connection query &optional (row-reader 'ignore-row-reader))
→ result

Sends the given query to the given connection, and interprets the results (if there are any) with the given row-reader. If the database returns information about the amount of rows affected, this is returned as a second value.

function prepare-query (database-connection name query)

Parse and plan the given query, and store it under the given name. Note that prepared statements are per-connection, so they can only be executed through the same connection that prepared them.

function exec-prepared (database-connection name parameters &optional (row-reader 'ignore-row-reader))
→ result

Execute the prepared statement by the given name. Parameters should be given as a list. Each value in this list should be of a type that to-sql-string has been specialised on. (Byte arrays will be passed in their binary form, without being put through to-sql-string.) The result of the executing the statement, if any, is interpreted by the given row reader, and returned. Again, the number or affected rows is optionally returned as a second value.

method to-sql-string (value)
→ (values string needs-escaping)

Convert a Lisp value to its textual unescaped SQL representation. Returns a second value indicating whether this value should be escaped if it is to be put directly into a query.

You can define to-sql-string methods for your own datatypes if you want to be able to pass them to exec-prepared. When a non-NIL second value is returned, this may be T to indicate that the first value should simply be escaped as a string, or a second string providing a type prefix for the value. (This is used by S-SQL.)

variable *silently-truncate-rationals*

When a rational number is passed into a query (as per to-sql-string), but it can not be expressed within 38 decimal digits (for example 1/3), it will be truncated, and lose some precision. Set this variable to nil to suppress that behaviour and raise an error instead.

variable *query-log*

When debugging, it can be helpful to inspect the queries that are being sent to the database. Set this variable to an output stream value (*standard-output*, for example) to have CL-postgres log every query it makes.

variable *query-callback*

When profiling or debugging, the *query-log* may not give enough information, or reparsing its output may not be feasible. This variable may be set to a designator of function taking two arguments. This function will be then called after every query, and receive query string and internal time units (as in (CL:GET-INTERNAL-REAL-TIME)) spent in query as its arguments.

Default value of this variable is 'LOG-QUERY, which takes care of *QUERY-LOG* processing. If you provide custom query callback and wish to keep *QUERY-LOG* functionality, you will have to call LOG-QUERY from your callback function

function log-query (query internal-time)

This function is default value of *QUERY-CALLBACK* and logs queries to *QUERY-LOG* if it is not NIL.

Reading values

CL-postgres knows how to convert commonly used PostgreSQL data types to Lisp values. This table shows the mapping:

double precisiondouble-float
bytea(vector (unsigned-byte 8))

The mapping from PostgreSQL types (identified by OID numbers) to the functions that interpret them is kept in so-called SQL readtables. All types for which no reader is defined will be returned as string values containing their PostgreSQL representation.

variable *sql-readtable*

This variable is used to choose the current readtable. For simple use, you will not have to touch this, but it is possible that code within a Lisp image requires different readers in different situations, in which case you can create separate read tables.

function copy-sql-readtable (table)
→ readtable

Copies a given readtable.

function default-sql-readtable ()
→ readtable

Returns the default readtable, containing only the readers defined by CL-postgres itself.

function set-sql-reader (oid function &key table binary-p)

Define a new reader for a given type. table defaults to *sql-readtable*. The reader function should take a single argument, a string, and transform that into some kind of equivalent Lisp value. When binary-p is true, the reader function is supposed to directly read the binary representation of the value. In most cases this is not recommended, but if you want to use it: provide a function that takes a binary input stream and an integer (the size of the value, in bytes), and reads the value from that stream. Note that reading less or more bytes than the given size will horribly break your connection.

function set-sql-datetime-readers (&key date timestamp timestamp-with-timezone time interval table)

Since there is no widely recognised standard way of represenging dates and times in Common Lisp, and reading these from string representation is clunky and slow, this function provides a way to easily plug in binary readers for the date, time, timestamp, and interval types. It should be given functions with the following signatures:

:date (days)
Where days is the amount of days since January 1st, 2000.
:timestamp (useconds)
Timestamps have a microsecond resolution. Again, the zero point is the start of the year 2000, UTC.
Like :timestamp, but for values of the 'timestamp with time zone' type (which PostgreSQL internally stores exactly the same as regular timestamps).
:time (useconds)
Refers to a time of day, counting from midnight.
:interval (months days useconds)
An interval is represented as several seperate components. The reason that days and microseconds are separated is that you might want to take leap seconds into account.

Row readers

Row readers are a way to read and group the results of queries. Roughly, they are functions that perform the iteration over the rows and cells in the result, and do something with the returned values.

macro row-reader ((fields) &body body)
→ function

Creates a row-reader, using the given name for the variable. Inside the body this variable refers to a vector of field descriptions. On top of that, two local functions are bound, next-row and next-field. The first will start reading the next row in the result, and returns a boolean indicating whether there is another row. The second will read and return one field, and should be passed the corresponding field description from the fields argument as a parameter.

A row reader should take care to iterate over all the rows in a result, and within each row iterate over all the fields. This means it should contain an outer loop that calls next-row, and every time next-row returns T it should iterate over the fields vector and call next-field for every field.

The definition of list-row-reader should give you an idea what a row reader looks like:

(row-reader (fields)
  (loop :while (next-row)
        :collect (loop :for field :across fields
                       :collect (next-field field))))

Obviously, row readers should not do things with the database connection like, say, close it or start a new query, since it still reading out the results from the current query.

macro def-row-reader (name (fields) &body body)

The defun-like variant of row-reader: creates a row reader and gives it a top-level function name.

method field-name (field)
→ string

This can be used to get information about the fields read by a row reader. Given a field description, it returns the name the database associated with this column.

method field-type (field)
→ oid

This extracts the PostgreSQL OID associated with this column. You can, if you really want to, query the pg_types table to find out more about the types denoted by OIDs.

function list-row-reader (socket fields)
→ list

A row reader that builds a list of lists from the query results.

function alist-row-reader (socket fields)
→ alist

A row reader that returns a list of alists, which associate column names with values.

function ignore-row-reader (socket fields)

A row reader that completely ignores the result of a query.


Opening or querying a database may raise errors. CL-postgres will wrap the errors that the server returns in a lisp condition, and raise conditions of the same type when it detects some problem itself. Socket errors are let through as they are.

condition database-error

The type of database-related conditions. For errors that you may want to catch by type, the cl-postgres-error package defines a bucket of subtypes used for specific errors. See the cl-postgres/package.lisp file for a list.

method database-error-message (database-error)
→ string

A short message associated with this error.

method database-error-detail (database-error)
→ string

A longer description of the problem, or NIL if none is available.

method database-error-code (database-error)
→ string

The error code PostgreSQL associated with this error, if any. See the PostgreSQL manual for their meaning.

method database-error-query (database-error)
→ string

The query that led to this error, or NIL if no query was involved.

method database-error-cause (database-error)
→ condition

The condition that caused this error, or NIL when it was not caused by another condition.

condition database-connection-error

Subtype of database-error. An error of this type (or one of its subclasses) is signaled when a query is attempted with a connection object that is no longer connected, or a database connection becomes invalid during a query. Always provides a :reconnect restart, which will cause the library to make an attempt to restore the connection and re-try the query.

The following shows an example use of this feature, a way to ensure that the first connection error causes a reconnect attempt, while others pass through as normal. A variation on this theme could continue trying to reconnect, with successively longer pauses.

(defun call-with-single-reconnect (fun)
  (let ((reconnected nil))
          (lambda (err)
            (when (not reconnected)
              (setf reconnected t)
              (invoke-restart :reconnect)))))
      (funcall fun))))