TidScan needs handling of a corner cases

Hi All,

I noticed that the TidScan fails to identify when the requested block is
not in the relation. Consider this (pg_class has 6 blocks):

postgres=# explain analyze select ctid, * from pg_class where ctid in (
'(6,1)' );
ERROR: could not read block 6 of relation 1663/11511/1259: read only 0 of
8192 bytes

Also, it is known that 0 is not a valid row-offset in the block, but the
tid input function accepts this value (it rejects -ve values). For the same
setup:

postgres=# explain analyze select ctid, * from pg_class where ctid in (
'(6,0)' );
QUERY
PLAN
----------------------------------------------------------------------------------------------------
Tid Scan on pg_class (cost=0.00..4.01 rows=1 width=211) (actual
time=0.009..0.009 rows=0 loops=1)
TID Cond: (ctid = '(6,0)'::tid)
Total runtime: 0.130 ms
(3 rows)

Can we safely fix these? First one by ignoring the request if
requested_block >= RelationGetNumberOfBlocks(), and second one by accepting
only non-zero positive values in the tid input function.

Best regards

--
gurjeet[(dot)singh](at)EnterpriseDB(dot)com
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