Let's say I have a table with fields A and B. I make regular queries on A+B, so I created a composite index on (A,B). Would queries on only A also be fully optimized by the composite index?
Additionally, I created an index on A, but Postgres still uses the composite index for queries on only A. If the previous answer is positive, I guess it doesn't really matter, but why does it select the composite index by default, if the single A index is available?
All of this applies to (default) B-tree indices, not other types.
A composite index on (a,b) supports every query that an index on just (a) would. The same is not true for (b,a) vs. (a). Related:
So an additional index on just (a) is redundant. Only the matter of index size remains.
Space is allocated in multiples of MAXALIGN, which is typically 8 bytes on a 64-bit OS or (much less common) 4 bytes on a 32-bit OS. If you are not sure, check pg_controldata. It also depends on data types of indexed columns (some require alignment padding) and actual content.
An index on, say, two integer columns (4 bytes each) typically ends up to be exactly as big as an index on just one, where another 4 bytes are lost to alignment padding.
Apart from that, there is really no downside for the query planner to use an index on (a,b) - compared to an index on just (a). And it is generally preferable for multiple queries to use the same index. The chance for it (or parts of it) to reside in (fast) cache grows when shared.
Since Postgres 13, index deduplication changes the picture gradually. Repeated index values can be compressed. An index on (a,b) is often less duplicative, so it gains less from deduplication. If a is a unique column, deduplication doesn't change much. (There can still be duplicative entries due to the MVCC model, only one visible to any transaction at the same time, so adding b can still have a minor effect.) There is a lot of fine print to when and how deduplication actually takes effect.
Coming from the opposite direction, when you need an additional index on (a,b), consider dropping an existing index on just (a). Often not possible for a PRIMARY KEY or UNIQUE constraint. Since Postgres 11 you might just append b to the constraint definition with an INCLUDE clause instead.
Or create the new index on (b,a) instead to cover queries on just b additionally. For only equality conditions the order of index expressions does not matter. It does, though, when involving range conditions. See:
There are potential downsides to including additional columns in indexes, even if that only uses space otherwise lost to alignment padding:
Whenever the additional column is updated, the index now needs an update, too, which might add cost to write operations and create more index bloat.
HOT updates ("Heap Only Tuple") on the table are not possible while any index column is involved. See:
How to measure object sizes:
According to your question you have a table with field A and B. If you your query is:
SELECT * FROM [YOUR TBL]
WHERE A='XXXX'
Optimizer will chose the Composite index to avoid Extract random access!
It is in the case if you just use just first in the predicate.
It will do scan if you use first columns of composite key and non-key column of composite key.
To trick it you can just dummy predicates like this and then non-key column:
[A,B] is your index, [C] - another column
To utilize index you write as:
SELECT
A,B,C,D,E
FROM
test
WHERE
A=1
AND
B=B
AND
C=3
...why does it select the composite index by default, if the single A index is available?
It will use index only in the case if there are one or two predicates [A] Or [A],[B]. It will not use it in the order [B],[A] or [A],[C]. To be able to utilize index with additional column [C], you need to enforce index by ordering predicates as [A],[B] and [C].
