Re: Signature change for SPI_cursor_open

Thomas Hallgren <thhal(at)mailblocks(dot)com> writes:
> Apparently the signature for function SPI_cursor_open got an additional
> "read_only" parameter starting with 8.0.0beta3. The documentation states
> that this flag should be "true for read-only execution".

> Is it correct to assume that this flag implies that some kind of
> optimization has been added and that if this flag is "false", the
> execution of the statement will take somewhat longer? I need some advice
> in how to handle this.

You should set the flag if and only if you are executing a pl/java
function that has a provolatile setting of "stable" or "immutable".
The new rule is that only functions declared "volatile" are allowed
to have side effects on the database. See pghackers discussions in
early September about updating snapshots, doing CommandCounterIncrement,
and so forth within functions.

For code examples see the commits in the pl languages here:

2004-09-13 16:07 tgl

* src/: include/executor/execdesc.h, include/executor/executor.h,
include/executor/spi.h, include/tcop/pquery.h,
include/tcop/utility.h, include/utils/tqual.h, pl/plperl/plperl.c,
pl/plperl/spi_internal.c, pl/plperl/spi_internal.h,
pl/plpgsql/src/pl_comp.c, pl/plpgsql/src/pl_exec.c,
pl/plpgsql/src/plpgsql.h, pl/plpython/plpython.c, pl/tcl/pltcl.c,
test/regress/expected/transactions.out,
test/regress/sql/transactions.sql: Redesign query-snapshot timing
so that volatile functions in READ COMMITTED mode see a fresh
snapshot for each command in the function, rather than using the
latest interactive command's snapshot. Also, suppress fresh
snapshots as well as CommandCounterIncrement inside STABLE and
IMMUTABLE functions, instead using the snapshot taken for the most
closely nested regular query. (This behavior is only sane for
read-only functions, so the patch also enforces that such functions
contain only SELECT commands.) As per my proposal of 6-Sep-2004; I
note that I floated essentially the same proposal on 19-Jun-2002,
but that discussion tailed off without any action. Since 8.0 seems
like the right place to be taking possibly nontrivial backwards
compatibility hits, let's get it done now.

regards, tom lane