DECLARE
DECLARE — define a cursor
Synopsis
DECLAREname[ BINARY ] [ ASENSITIVE | INSENSITIVE ] [ [ NO ] SCROLL ] CURSOR [ { WITH | WITHOUT } HOLD ] FORquery
Description
DECLARE allows a user to create cursors, which can be used to retrieve a small number of rows at a time out of a larger query. After the cursor is created, rows are fetched from it usingFETCH.
Note
This page describes usage of cursors at the SQL command level. If you are trying to use cursors inside aPL/pgSQL function, the rules are different — seeSection 41.7.
Parameters
nameThe name of the cursor to be created. This must be different from any other active cursor name in the session.
BINARYCauses the cursor to return data in binary rather than in text format.
ASENSITIVEINSENSITIVECursor sensitivity determines whether changes to the data underlying the cursor, done in the same transaction, after the cursor has been declared, are visible in the cursor.
INSENSITIVEmeans they are not visible,ASENSITIVEmeans the behavior is implementation-dependent. A third behavior,SENSITIVE, meaning that such changes are visible in the cursor, is not available inPostgres Pro. InPostgres Pro, all cursors are insensitive; so these key words have no effect and are only accepted for compatibility with the SQL standard.Specifying
INSENSITIVEtogether withFOR UPDATEorFOR SHAREis an error.SCROLLNO SCROLLSCROLLspecifies that the cursor can be used to retrieve rows in a nonsequential fashion (e.g., backward). Depending upon the complexity of the query's execution plan, specifyingSCROLLmight impose a performance penalty on the query's execution time.NO SCROLLspecifies that the cursor cannot be used to retrieve rows in a nonsequential fashion. The default is to allow scrolling in some cases; this is not the same as specifyingSCROLL. SeeNotes below for details.WITH HOLDWITHOUT HOLDWITH HOLDspecifies that the cursor can continue to be used after the transaction that created it successfully commits.WITHOUT HOLDspecifies that the cursor cannot be used outside of the transaction that created it. If neitherWITHOUT HOLDnorWITH HOLDis specified,WITHOUT HOLDis the default.queryA
SELECTorVALUEScommand which will provide the rows to be returned by the cursor.
The key wordsASENSITIVE,BINARY,INSENSITIVE, andSCROLL can appear in any order.
Notes#
Normal cursors return data in text format, the same as aSELECT would produce. TheBINARY option specifies that the cursor should return data in binary format. This reduces conversion effort for both the server and client, at the cost of more programmer effort to deal with platform-dependent binary data formats. As an example, if a query returns a value of one from an integer column, you would get a string of1 with a default cursor, whereas with a binary cursor you would get a 4-byte field containing the internal representation of the value (in big-endian byte order).
Binary cursors should be used carefully. Many applications, includingpsql, are not prepared to handle binary cursors and expect data to come back in the text format.
Note
When the client application uses the“extended query” protocol to issue aFETCH command, the Bind protocol message specifies whether data is to be retrieved in text or binary format. This choice overrides the way that the cursor is defined. The concept of a binary cursor as such is thus obsolete when using extended query protocol — any cursor can be treated as either text or binary.
UnlessWITH HOLD is specified, the cursor created by this command can only be used within the current transaction. Thus,DECLARE withoutWITH HOLD is useless outside a transaction block: the cursor would survive only to the completion of the statement. ThereforePostgres Pro reports an error if such a command is used outside a transaction block. UseBEGIN andCOMMIT (orROLLBACK) to define a transaction block.
IfWITH HOLD is specified and the transaction that created the cursor successfully commits, the cursor can continue to be accessed by subsequent transactions in the same session. (But if the creating transaction is aborted, the cursor is removed.) A cursor created withWITH HOLD is closed when an explicitCLOSE command is issued on it, or the session ends. In the current implementation, the rows represented by a held cursor are copied into a temporary file or memory area so that they remain available for subsequent transactions.
WITH HOLD may not be specified when the query includesFOR UPDATE orFOR SHARE.
TheSCROLL option should be specified when defining a cursor that will be used to fetch backwards. This is required by the SQL standard. However, for compatibility with earlier versions,Postgres Pro will allow backward fetches withoutSCROLL, if the cursor's query plan is simple enough that no extra overhead is needed to support it. However, application developers are advised not to rely on using backward fetches from a cursor that has not been created withSCROLL. IfNO SCROLL is specified, then backward fetches are disallowed in any case.
Backward fetches are also disallowed when the query includesFOR UPDATE orFOR SHARE; thereforeSCROLL may not be specified in this case.
Caution
Scrollable cursors may give unexpected results if they invoke any volatile functions (seeSection 36.7). When a previously fetched row is re-fetched, the functions might be re-executed, perhaps leading to results different from the first time. It's best to specifyNO SCROLL for a query involving volatile functions. If that is not practical, one workaround is to declare the cursorSCROLL WITH HOLD and commit the transaction before reading any rows from it. This will force the entire output of the cursor to be materialized in temporary storage, so that volatile functions are executed exactly once for each row.
If the cursor's query includesFOR UPDATE orFOR SHARE, then returned rows are locked at the time they are first fetched, in the same way as for a regularSELECT command with these options. In addition, the returned rows will be the most up-to-date versions.
Caution
It is generally recommended to useFOR UPDATE if the cursor is intended to be used withUPDATE ... WHERE CURRENT OF orDELETE ... WHERE CURRENT OF. UsingFOR UPDATE prevents other sessions from changing the rows between the time they are fetched and the time they are updated. WithoutFOR UPDATE, a subsequentWHERE CURRENT OF command will have no effect if the row was changed since the cursor was created.
Another reason to useFOR UPDATE is that without it, a subsequentWHERE CURRENT OF might fail if the cursor query does not meet the SQL standard's rules for being“simply updatable” (in particular, the cursor must reference just one table and not use grouping orORDER BY). Cursors that are not simply updatable might work, or might not, depending on plan choice details; so in the worst case, an application might work in testing and then fail in production. IfFOR UPDATE is specified, the cursor is guaranteed to be updatable.
The main reason not to useFOR UPDATE withWHERE CURRENT OF is if you need the cursor to be scrollable, or to be isolated from concurrent updates (that is, continue to show the old data). If this is a requirement, pay close heed to the caveats shown above.
The SQL standard only makes provisions for cursors in embeddedSQL. ThePostgres Pro server does not implement an The server data structure underlying an open cursor is called aportal. Portal names are exposed in the client protocol: a client can fetch rows directly from an open portal, if it knows the portal name. When creating a cursor with You can see all available cursors by querying theOPEN statement for cursors; a cursor is considered to be open when it is declared. However,ECPG, the embedded SQL preprocessor forPostgres Pro, supports the standard SQL cursor conventions, including those involvingDECLARE andOPEN statements.DECLARE, the portal name is the same as the cursor name.pg_cursors system view.
Examples
To declare a cursor:
DECLARE liahona CURSOR FOR SELECT * FROM films;
SeeFETCH for more examples of cursor usage.
Compatibility
The SQL standard allows cursors only in embeddedSQL and in modules.Postgres Pro permits cursors to be used interactively. According to the SQL standard, changes made to insensitive cursors by Binary cursors are aPostgres Pro extension.UPDATE ... WHERE CURRENT OF andDELETE ... WHERE CURRENT OF statements are visible in that same cursor.Postgres Pro treats these statements like all other data changing statements in that they are not visible in insensitive cursors.