Re: Porting MSSQL to PGSQL (Was: [OT] MySQL is bad, but

Tom Lane wrote:

>"Jim C. Nasby" <jnasby@pervasive.com> writes:
>  
>
>>If there was more information than the tuplestore could keep in memory,
>>then a TIDstore might be faster, but only if it resulted in reading from
>>the heap sequentially, or very near it.
>>    
>>
>
>That's easily arranged, use a bitmap indexing data structure.
>
>I think we could probably even live with the structure becoming lossy
>under memory pressure: AFAICS, all rows modified by a single query ought
>to have the same XMIN/CMIN (or XMAX/CMAX for deleted rows), so it should
>be possible to verify whether a particular row is one of the interesting
>ones or not.
>
>I think the hard part of this task is designing the API for access to
>the rowsets from triggers.
>
>
>  
>

How expensive is this going to be, especially for huge numbers of rows? 
Would it be done for all queries, or just those with a per statement 
trigger, or only when explicitly requested?

cheers

andrew