Hello,
On 2018-01-29 18:08, Stephan Buchert wrote:
But then I have a related question: to get my hands on each row in SELECTs
with GROUP BY I could write an aggregate extension function. How do I see
there, when a group ends and a new one starts? I.e. How do I implement the
xStep and xFinal C functions as requested at
https://sqlite.org/c3ref/create_function.html
SQLite manages an area of memory called ``aggregate context''. When you
call ``sqlite3_aggregate_context()'' from your ``xStep'' and/or
``xFinal'' then you will receive a pointer to that memory area, which is
kept as long as an invocation of ``xStep''/``xFinal'' considers the same
group of GROUP BY:
typedef struct myAggCtx {
int notTheFirstTime;
/* Other state variables follow */
} myAggCtx;
xStep(ctx, ...)
{
myAggCtx *actx = sqlite3_aggregate_context(ctx, sizeof(myAggCtx));
if ( 0 == actx->notTheFirstTime ) {
/* Do your starting of group stuff */
actx->notTheFirstTime = !0;
}
/* ... */
}
xFinal(ctx, ...)
myAggCtx *actx = sqlite3_aggregate_context(ctx, sizeof(myAggCtx));
if ( 0 == actx->notTheFirstTime ) {
/* First time allocated context so there was no records in the group */
}
/* ... */
}
Beware of passing the same number of bytes each time, because
``sqlite3_aggregate_context()'' does not reallocates. If an aggregate
context has been allocated previously, then
``sqlite3_aggregate_context()'' will returns the same memory area
regardless of subsequent numbers of bytes passed. I.e.
sqlite3_aggregate_context(ctx, 1) will return 1byte size memory --
subsequent call (while in the same group of GROUP BY)
sqlite3_aggregate_context(ctx, 1000000) will return the same 1byte size
memory.
-- best regards
Cezary H. Noweta
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