'\" t .\" Title: PREPARE .\" Author: The PostgreSQL Global Development Group .\" Generator: DocBook XSL Stylesheets vsnapshot .\" Date: 2021 .\" Manual: PostgreSQL 13.4 Documentation .\" Source: PostgreSQL 13.4 .\" Language: English .\" .TH "PREPARE" "7" "2021" "PostgreSQL 13.4" "PostgreSQL 13.4 Documentation" .\" ----------------------------------------------------------------- .\" * Define some portability stuff .\" ----------------------------------------------------------------- .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .\" http://bugs.debian.org/507673 .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .ie \n(.g .ds Aq \(aq .el .ds Aq ' .\" ----------------------------------------------------------------- .\" * set default formatting .\" ----------------------------------------------------------------- .\" disable hyphenation .nh .\" disable justification (adjust text to left margin only) .ad l .\" ----------------------------------------------------------------- .\" * MAIN CONTENT STARTS HERE * .\" ----------------------------------------------------------------- .SH "NAME" PREPARE \- prepare a statement for execution .SH "SYNOPSIS" .sp .nf PREPARE \fIname\fR [ ( \fIdata_type\fR [, \&.\&.\&.] ) ] AS \fIstatement\fR .fi .SH "DESCRIPTION" .PP \fBPREPARE\fR creates a prepared statement\&. A prepared statement is a server\-side object that can be used to optimize performance\&. When the \fBPREPARE\fR statement is executed, the specified statement is parsed, analyzed, and rewritten\&. When an \fBEXECUTE\fR command is subsequently issued, the prepared statement is planned and executed\&. This division of labor avoids repetitive parse analysis work, while allowing the execution plan to depend on the specific parameter values supplied\&. .PP Prepared statements can take parameters: values that are substituted into the statement when it is executed\&. When creating the prepared statement, refer to parameters by position, using $1, $2, etc\&. A corresponding list of parameter data types can optionally be specified\&. When a parameter\*(Aqs data type is not specified or is declared as unknown, the type is inferred from the context in which the parameter is first referenced (if possible)\&. When executing the statement, specify the actual values for these parameters in the \fBEXECUTE\fR statement\&. Refer to \fBEXECUTE\fR(7) for more information about that\&. .PP Prepared statements only last for the duration of the current database session\&. When the session ends, the prepared statement is forgotten, so it must be recreated before being used again\&. This also means that a single prepared statement cannot be used by multiple simultaneous database clients; however, each client can create their own prepared statement to use\&. Prepared statements can be manually cleaned up using the \fBDEALLOCATE\fR(7) command\&. .PP Prepared statements potentially have the largest performance advantage when a single session is being used to execute a large number of similar statements\&. The performance difference will be particularly significant if the statements are complex to plan or rewrite, e\&.g\&., if the query involves a join of many tables or requires the application of several rules\&. If the statement is relatively simple to plan and rewrite but relatively expensive to execute, the performance advantage of prepared statements will be less noticeable\&. .SH "PARAMETERS" .PP \fIname\fR .RS 4 An arbitrary name given to this particular prepared statement\&. It must be unique within a single session and is subsequently used to execute or deallocate a previously prepared statement\&. .RE .PP \fIdata_type\fR .RS 4 The data type of a parameter to the prepared statement\&. If the data type of a particular parameter is unspecified or is specified as unknown, it will be inferred from the context in which the parameter is first referenced\&. To refer to the parameters in the prepared statement itself, use $1, $2, etc\&. .RE .PP \fIstatement\fR .RS 4 Any \fBSELECT\fR, \fBINSERT\fR, \fBUPDATE\fR, \fBDELETE\fR, or \fBVALUES\fR statement\&. .RE .SH "NOTES" .PP A prepared statement can be executed with either a generic plan or a custom plan\&. A generic plan is the same across all executions, while a custom plan is generated for a specific execution using the parameter values given in that call\&. Use of a generic plan avoids planning overhead, but in some situations a custom plan will be much more efficient to execute because the planner can make use of knowledge of the parameter values\&. (Of course, if the prepared statement has no parameters, then this is moot and a generic plan is always used\&.) .PP By default (that is, when plan_cache_mode is set to auto), the server will automatically choose whether to use a generic or custom plan for a prepared statement that has parameters\&. The current rule for this is that the first five executions are done with custom plans and the average estimated cost of those plans is calculated\&. Then a generic plan is created and its estimated cost is compared to the average custom\-plan cost\&. Subsequent executions use the generic plan if its cost is not so much higher than the average custom\-plan cost as to make repeated replanning seem preferable\&. .PP This heuristic can be overridden, forcing the server to use either generic or custom plans, by setting \fIplan_cache_mode\fR to force_generic_plan or force_custom_plan respectively\&. This setting is primarily useful if the generic plan\*(Aqs cost estimate is badly off for some reason, allowing it to be chosen even though its actual cost is much more than that of a custom plan\&. .PP To examine the query plan PostgreSQL is using for a prepared statement, use \fBEXPLAIN\fR(7), for example .sp .if n \{\ .RS 4 .\} .nf EXPLAIN EXECUTE \fIname\fR(\fIparameter_values\fR); .fi .if n \{\ .RE .\} .sp If a generic plan is in use, it will contain parameter symbols $\fIn\fR, while a custom plan will have the supplied parameter values substituted into it\&. .PP For more information on query planning and the statistics collected by PostgreSQL for that purpose, see the \fBANALYZE\fR(7) documentation\&. .PP Although the main point of a prepared statement is to avoid repeated parse analysis and planning of the statement, PostgreSQL will force re\-analysis and re\-planning of the statement before using it whenever database objects used in the statement have undergone definitional (DDL) changes since the previous use of the prepared statement\&. Also, if the value of search_path changes from one use to the next, the statement will be re\-parsed using the new \fIsearch_path\fR\&. (This latter behavior is new as of PostgreSQL 9\&.3\&.) These rules make use of a prepared statement semantically almost equivalent to re\-submitting the same query text over and over, but with a performance benefit if no object definitions are changed, especially if the best plan remains the same across uses\&. An example of a case where the semantic equivalence is not perfect is that if the statement refers to a table by an unqualified name, and then a new table of the same name is created in a schema appearing earlier in the \fIsearch_path\fR, no automatic re\-parse will occur since no object used in the statement changed\&. However, if some other change forces a re\-parse, the new table will be referenced in subsequent uses\&. .PP You can see all prepared statements available in the session by querying the pg_prepared_statements system view\&. .SH "EXAMPLES" .PP Create a prepared statement for an \fBINSERT\fR statement, and then execute it: .sp .if n \{\ .RS 4 .\} .nf PREPARE fooplan (int, text, bool, numeric) AS INSERT INTO foo VALUES($1, $2, $3, $4); EXECUTE fooplan(1, \*(AqHunter Valley\*(Aq, \*(Aqt\*(Aq, 200\&.00); .fi .if n \{\ .RE .\} .PP Create a prepared statement for a \fBSELECT\fR statement, and then execute it: .sp .if n \{\ .RS 4 .\} .nf PREPARE usrrptplan (int) AS SELECT * FROM users u, logs l WHERE u\&.usrid=$1 AND u\&.usrid=l\&.usrid AND l\&.date = $2; EXECUTE usrrptplan(1, current_date); .fi .if n \{\ .RE .\} .sp In this example, the data type of the second parameter is not specified, so it is inferred from the context in which $2 is used\&. .SH "COMPATIBILITY" .PP The SQL standard includes a \fBPREPARE\fR statement, but it is only for use in embedded SQL\&. This version of the \fBPREPARE\fR statement also uses a somewhat different syntax\&. .SH "SEE ALSO" \fBDEALLOCATE\fR(7), \fBEXECUTE\fR(7)