Re: LIMIT in DECLARE CURSOR: request for comments

At 12:18 27/10/00 -0400, Tom Lane wrote:
>
>1. If DECLARE CURSOR does not contain a LIMIT, continue to plan on the
>basis of 10%-or-so fetch (I'd consider anywhere from 5% to 25% to be
>just as reasonable, if people want to argue about the exact number;
>perhaps a SET variable is in order?). 10% seems to be a reasonable
>compromise between delivering tuples promptly and not choosing a plan
>that will take forever if the user fetches the whole result.

SET sounds good; will this work on a per-connection basis?

>2. If DECLARE CURSOR contains a specific "LIMIT n" clause, plan on
>the assumption that n tuples will be fetched. For small n this allows
>the user to heavily bias the plan towards fast start. Since the LIMIT
>will actually be enforced by the executor, the user cannot bias the
>plan more heavily than is justified by the number of tuples he's
>intending to fetch, however.

Fine.

>3. If DECLARE CURSOR contains "LIMIT ALL", plan on the assumption that
>all tuples will be fetched, ie, select lowest-total-cost plan.

Good.

>
>Comments?
>

I don't suppose you'd consider 'OPTIMIZE FOR TOTAL COST' and 'OPTIMIZE FOR
FAST START' optimizer hints?

Also, does the change you have made to the executor etc mean that
subselect-with-limit is now possible?

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