Hash join, how to know how much of pga is using?

Question

Good Day everybody...
Is there a way to know if i have enough pga for a given hash join??'
so far i have PGA_AGGREGATE_TARGET = 200MB
WORK AREA SIZE POLICY = AUTO
The advisor from EM looks good, but i know that the query im interested on its expensive.
so reworking the question...
¿how can i get the propper ammount of bytes needed by a given hash join?

Answer 1

Its complicated - There are pages of stuff in the manual, histograms, forecast histograms.
But I think you can get a long way with:
SELECT name profile, cnt, decode(total, 0, 0, round(cnt*100/total)) percentage
FROM (SELECT name, value cnt, (sum(value) over ()) total
FROM V$SYSSTAT
WHERE name like 'workarea exec%');
The output of this query might look like the following:
PROFILE CNT PERCENTAGE
workarea executions - optimal 5395 95
workarea executions - onepass 284 5
workarea executions - multipass 0 0
If all executions are optimal, who can ask for more?
Also:
V$PGA_TARGET_ADVICE
This view predicts how the statistics cache hit percentage and over allocation count in V$PGASTAT will be impacted if you change the value of the initialization parameter PGA_AGGREGATE_TARGET. Example 14-8 shows a typical query of this view:
Example 14-8 Querying V$PGA_TARGET_ADVICE
SELECT round(PGA_TARGET_FOR_ESTIMATE/1024/1024) target_mb,
ESTD_PGA_CACHE_HIT_PERCENTAGE cache_hit_perc,
ESTD_OVERALLOC_COUNT
FROM V$PGA_TARGET_ADVICE;
The output of this query might look like the following:
TARGET_MB CACHE_HIT_PERC ESTD_OVERALLOC_COUNT
63 23 367
125 24 30
250 30 3
375 39 0
500 58 0
600 59 0
700 59 0
800 60 0
900 60 0
1000 61 0
1500 67 0
2000 76 0
3000 83 0
4000 85 0
As far as I understand if, if the overallocation count is nonzero, Oracle will override your pga_aggregate_target anyway. (Having said that, dont forget, this parameter is a target, not a hard & fast limit)