# pgr_primDFS¶

pgr_primDFS — Prim algorithm for Minimum Spanning Tree with Depth First Search ordering.

Availability

• Version 3.0.0
• New Official function

Support

## Description¶

Visits and extracts the nodes information in Depth First Search ordering of the Minimum Spanning Tree created using Prims’s algorithm.

The main Characteristics are:

• It’s implementation is only on undirected graph.
• Process is done only on edges with positive costs.
• When the graph is connected
• The resulting edges make up a tree
• When the graph is not connected,
• Finds a minimum spanning tree for each connected component.
• The resulting edges make up a forest.
• Prim’s running time: $$O(E*log V)$$
• Returned tree nodes from a root vertex are on Depth First Search order
• Depth First Search Running time: $$O(E + V)$$

## Signatures¶

pgr_primDFS(Edges SQL, Root vid [, max_depth])
pgr_primDFS(Edges SQL, Root vids [, max_depth])

RETURNS SET OF (seq, depth, start_vid, node, edge, cost, agg_cost)


### Single vertex¶

pgr_primDFS(Edges SQL, Root vid [, max_depth])

RETURNS SET OF (seq, depth, start_vid, node, edge, cost, agg_cost)

Example: The Minimum Spanning Tree having as root vertex $$2$$
SELECT * FROM pgr_primDFS(
'SELECT id, source, target, cost, reverse_cost FROM edge_table ORDER BY id',
2
);
seq | depth | start_vid | node | edge | cost | agg_cost
-----+-------+-----------+------+------+------+----------
1 |     0 |         2 |    2 |   -1 |    0 |        0
2 |     1 |         2 |    1 |    1 |    1 |        1
3 |     1 |         2 |    3 |    2 |    1 |        1
4 |     2 |         2 |    4 |    3 |    1 |        2
5 |     2 |         2 |    6 |    5 |    1 |        2
6 |     3 |         2 |    9 |    9 |    1 |        3
7 |     3 |         2 |   11 |   11 |    1 |        3
8 |     4 |         2 |   12 |   13 |    1 |        4
9 |     1 |         2 |    5 |    4 |    1 |        1
10 |     2 |         2 |    8 |    7 |    1 |        2
11 |     3 |         2 |    7 |    6 |    1 |        3
12 |     2 |         2 |   10 |   10 |    1 |        2
13 |     3 |         2 |   13 |   14 |    1 |        3
(13 rows)



### Multiple vertices¶

pgr_primDFS(Edges SQL, Root vids [, max_depth])

RETURNS SET OF (seq, depth, start_vid, node, edge, cost, agg_cost)

Example: The Minimum Spanning Tree starting on vertices $$\{13, 2\}$$ with $$depth <= 3$$
SELECT * FROM pgr_primDFS(
'SELECT id, source, target, cost, reverse_cost FROM edge_table ORDER BY id',
ARRAY[13,2], max_depth := 3
);
seq | depth | start_vid | node | edge | cost | agg_cost
-----+-------+-----------+------+------+------+----------
1 |     0 |         2 |    2 |   -1 |    0 |        0
2 |     1 |         2 |    1 |    1 |    1 |        1
3 |     1 |         2 |    3 |    2 |    1 |        1
4 |     2 |         2 |    4 |    3 |    1 |        2
5 |     2 |         2 |    6 |    5 |    1 |        2
6 |     3 |         2 |    9 |    9 |    1 |        3
7 |     3 |         2 |   11 |   11 |    1 |        3
8 |     1 |         2 |    5 |    4 |    1 |        1
9 |     2 |         2 |    8 |    7 |    1 |        2
10 |     3 |         2 |    7 |    6 |    1 |        3
11 |     2 |         2 |   10 |   10 |    1 |        2
12 |     3 |         2 |   13 |   14 |    1 |        3
13 |     0 |        13 |   13 |   -1 |    0 |        0
14 |     1 |        13 |   10 |   14 |    1 |        1
15 |     2 |        13 |    5 |   10 |    1 |        2
16 |     3 |        13 |    2 |    4 |    1 |        3
17 |     3 |        13 |    8 |    7 |    1 |        3
(17 rows)



## Parameters¶

Parameter Type Description
Edges SQL TEXT SQL query described in Inner query.
Root vid BIGINT

Identifier of the root vertex of the tree.

• Used on Single vertex
• When value is $$0$$ then gets the spanning forest starting in aleatory nodes for each tree in the forest.
Root vids ARRAY[ANY-INTEGER]

Array of identifiers of the root vertices.

• Used on Multiple vertices
• $$0$$ values are ignored
• For optimization purposes, any duplicated value is ignored.

### Optional Parameters¶

Parameter Type Default Description
max_depth BIGINT $$9223372036854775807$$

Upper limit for depth of node in the tree

• When value is Negative then throws error

## Inner query¶

Column Type Default Description
id ANY-INTEGER   Identifier of the edge.
source ANY-INTEGER   Identifier of the first end point vertex of the edge.
target ANY-INTEGER   Identifier of the second end point vertex of the edge.
cost ANY-NUMERICAL

Weight of the edge (source, target)

• When negative: edge (source, target) does not exist, therefore it’s not part of the graph.
reverse_cost ANY-NUMERICAL -1

Weight of the edge (target, source),

• When negative: edge (target, source) does not exist, therefore it’s not part of the graph.

Where:

ANY-INTEGER: SMALLINT, INTEGER, BIGINT SMALLINT, INTEGER, BIGINT, REAL, FLOAT

## Result Columns¶

Returns SET OF (seq, depth, start_vid, node, edge, cost, agg_cost)

Column Type Description
seq BIGINT Sequential value starting from $$1$$.
depth BIGINT

Depth of the node.

• $$0$$ when node = start_vid.
start_vid BIGINT

Identifier of the root vertex.

node BIGINT Identifier of node reached using edge.
edge BIGINT

Identifier of the edge used to arrive to node.

• $$-1$$ when node = start_vid.
cost FLOAT Cost to traverse edge.
agg_cost FLOAT Aggregate cost from start_vid to node.