Thomas Hallgren <thhal@mailblocks.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:
Redesignquery-snapshot timingso that volatile functions in READ COMMITTED mode see a freshsnapshot for each command in
thefunction, rather than using thelatest interactive command's snapshot. Also, suppress freshsnapshots as well as
CommandCounterIncrementinside STABLE andIMMUTABLE functions, instead using the snapshot taken for the mostclosely
nestedregular query. (This behavior is only sane forread-only functions, so the patch also enforces that such
functionscontainonly SELECT commands.) As per my proposal of 6-Sep-2004; Inote that I floated essentially the same
proposalon 19-Jun-2002,but that discussion tailed off without any action. Since 8.0 seemslike the right place to be
takingpossibly nontrivial backwardscompatibility hits, let's get it done now.
regards, tom lane