problem solved twice

[Daniela]

I am using Cplex to solve the Set Partitioning Problem from your documentation.

https://coin-or.github.io/pulp/CaseStudies/a_set_partitioning_problem.html

I receive the following output, which shows that the problem is solved using Branch & Cut first and after that the "MILP problem relaxed to LP with fixed integer variables using

incumbent solution" is solved again using dual simplex. Why is it solved twice? I'm calling Cplex via "prob.solve(CPLEX_CMD(msg=1)).

Welcome to IBM(R) ILOG(R) CPLEX(R) Interactive Optimizer 12.10.0.0

  with Simplex, Mixed Integer & Barrier Optimizers

5725-A06 5725-A29 5724-Y48 5724-Y49 5724-Y54 5724-Y55 5655-Y21

Copyright IBM Corp. 1988, 2019.  All Rights Reserved.

Type 'help' for a list of available commands.

Type 'help' followed by a command name for more

information on commands.

CPLEX> Problem '/var/folders/rm/ncy_nf_932l2h58hjq1vjddm0000gn/T/7fd8c3f1c9a54deb9a30af67efede4cf-pulp.lp' read.

Read time = 0.01 sec. (0.29 ticks)

CPLEX> Version identifier: 12.10.0.0 | 2019-11-26 | 843d4de

Tried aggregator 1 time.

MIP Presolve modified 697 coefficients.

Reduced MIP has 18 rows, 3213 columns, and 14365 nonzeros.

Reduced MIP has 3213 binaries, 0 generals, 0 SOSs, and 0 indicators.

Presolve time = 0.03 sec. (24.12 ticks)

Found incumbent of value 44.000000 after 0.08 sec. (54.94 ticks)

Tried aggregator 1 time.

Detecting symmetries...

Reduced MIP has 18 rows, 3213 columns, and 14365 nonzeros.

Reduced MIP has 3213 binaries, 0 generals, 0 SOSs, and 0 indicators.

Presolve time = 0.02 sec. (23.18 ticks)

Probing time = 0.02 sec. (7.05 ticks)

Clique table members: 17.

MIP emphasis: balance optimality and feasibility.

MIP search method: dynamic search.

Parallel mode: deterministic, using up to 4 threads.

Root relaxation solution time = 0.01 sec. (3.92 ticks)

        Nodes                                         Cuts/

   Node  Left     Objective  IInf  Best Integer    Best Bound    ItCnt     Gap

*     0+    0                           44.0000        0.0000           100.00%

*     0     0      integral     0       12.0000       12.0000       28    0.00%

Elapsed time = 0.14 sec. (93.80 ticks, tree = 0.00 MB, solutions = 2)

Root node processing (before b&c):

  Real time             =    0.14 sec. (93.92 ticks)

Parallel b&c, 4 threads:

  Real time             =    0.00 sec. (0.00 ticks)

  Sync time (average)   =    0.00 sec.

  Wait time (average)   =    0.00 sec.

                          ------------

Total (root+branch&cut) =    0.14 sec. (93.92 ticks)

Solution pool: 2 solutions saved.

MIP - Integer optimal solution:  Objective =  1.2000000000e+01

Solution time =    0.14 sec.  Iterations = 28  Nodes = 0

Deterministic time = 93.93 ticks  (654.94 ticks/sec)

CPLEX> MILP problem relaxed to LP with fixed integer variables using

incumbent solution.

CPLEX> Version identifier: 12.10.0.0 | 2019-11-26 | 843d4de

Parallel mode: deterministic, using up to 4 threads for concurrent optimization:

 * Starting dual Simplex on 1 thread...

 * Starting Barrier on 3 threads...

Tried aggregator 1 time.

LP Presolve eliminated 18 rows and 3213 columns.

All rows and columns eliminated.

Presolve time = 0.00 sec. (1.26 ticks)

Dual simplex solved model.

Dual simplex - Optimal:  Objective =  1.2000000000e+01

Solution time =    0.01 sec.  Iterations = 0 (0)

Deterministic time = 1.71 ticks  (307.89 ticks/sec)

CPLEX> Solution written to file '/var/folders/rm/ncy_nf_932l2h58hjq1vjddm0000gn/T/7fd8c3f1c9a54deb9a30af67efede4cf-pulp.sol'.

CPLEX> The maximum happiness is:  12.0

The table order is:

('M', 'N')

('E', 'F', 'G')

('A', 'B', 'C', 'D')

('I', 'J', 'K', 'L')

('O', 'P', 'Q', 'R')

Any help is welcome, thanks in advance.