Since PostgreSQL requires that cursors be defined The INSENSITIVE SQL keyword exists to ensure that all data retrieved from theĬursor remains unchanged from other cursors or connections. The INSENSITIVE and SCROLL keywords exist forĬompliance with the SQL standard, though they each define PostgreSQL's default behavior and are never necessary. Of standard ASCII this can be more efficient, though it is only relevant to custom applications, as clients suchĪs psql are not built to handle anything but text output. The optional BINARY keyword causes output to be retrieved in binary format instead MOVE command moves the "current" location of the cursor within the result set, and theĬLOSE command closes the cursor, freeing up any associated memory.Ĭursorname is the name of the cursor to create. TheįETCH command lets you pull rows from an open cursor. In memory, and then populating the cursor with information about the result set returned from the executed query. The DECLARE command both defines and opens a cursor, in effect defining the cursor The four SQLĬommands involved with PostgreSQL cursors are DECLARE, This section uses psql to demonstrate the fundamental concepts of cursors with SQL. This prevents having to store all of the results in memory within theĬursors are often abstracted within a programming API (such as libpq++'s PgCursorĬlass), though they can also be directly created and manipulated through standard SQL commands. Used within a programming Application Programming Interface (API), cursors are often used toĪllow multiple queries to be executed to a single database backend, which are then tracked and managed separately by theĪpplication through references to the cursor. By executing a cursor, and maintaining a reference to its returned result set, an application can moreĮfficiently manage which rows to retrieve from a result set at different times, without having to re-execute the query with Cursors are typically used within applications that maintain a persistent connection to the
Postgresql for loop cursor update#
Or to put it the other way round: doing set-based operations (like my single update example) is always the better choice.A SQL cursor in PostgreSQL is a read-only pointer to a fully executed SELECT Row-by-row processing is an anti-pattern that will almost always have bad performance. Relational databases are made to deal with "sets" of data. WHERE id = r.id - note the where condition that uses the value from the record variableīut again: if your table is "huge" as you say, the loop is an extremely bad solution. r is a structure that contains an element for each column in the select list If you really, really want to do it inefficiently in a loop, the your function should look something like this ( not tested!): CREATE OR REPLACE FUNCTION some_function() You are also retrieving the values incorrectly from the cursor.
So even if your update in the loop would find a row, it would update it to NULL. As C is NULL initially, b*c will also yield null. Your function also seems to use the wrong formula: when id = 'a' THEN B*C I guess that should be: then a*b. You don't have a row with an empty string in the column id. The reason your function isn't doing anything is this: update table set C = result No need for a loop or a function, this can be done with a single update statement: update table_name If anyone could point me in the right direction or point out anything blatantly obvious that I'm doing wrong I'd much appreciate it. But as I only want to update existing rows should I need to return anything? There's probably easier ways of doing this than using a loop but I'd like to get it working using this method. I'm sure there's something silly i'm missing, probably around what I'm, returning. r integer not too sure if this needs to be declared or not DROP FUNCTION some_function() ĬREATE OR REPLACE FUNCTION some_function() The function I've written (which isn't giving any errors but also isn't updating anything either) looks like this. So ultimately I'd like to loop through all the rows of the table and update column C according to the value in the "id" column. This would then give me a table like +-+-+-+ On the values in columns a and b and then inserts a new value in c.įor example, where id = a, update table set C = A*B, or where id = d set C = A + B etc. I want to write a for loop which iterates through all of the rows and does some operation The table i'm working on is huge so for brevity i'll give a smaller example which should get the point across. I'm new enough to postgresql, and I'm having issues updating a column of null values in a table using a for loop.
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